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804 lines
31 KiB
TeX
804 lines
31 KiB
TeX
\section{Built-in Types}
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The following sections describe the standard types that are built into
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the interpreter. These are the numeric types, sequence types, and
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several others, including types themselves. There is no explicit
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Boolean type; use integers instead.
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\indexii{built-in}{types}
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\indexii{Boolean}{type}
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Some operations are supported by several object types; in particular,
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all objects can be compared, tested for truth value, and converted to
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a string (with the \code{`{\rm \ldots}`} notation). The latter conversion is
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implicitly used when an object is written by the \code{print} statement.
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\stindex{print}
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\subsection{Truth Value Testing}
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Any object can be tested for truth value, for use in an \code{if} or
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\code{while} condition or as operand of the Boolean operations below.
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The following values are considered false:
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\stindex{if}
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\stindex{while}
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\indexii{truth}{value}
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\indexii{Boolean}{operations}
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\index{false}
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\begin{itemize}
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\renewcommand{\indexsubitem}{(Built-in object)}
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\item \code{None}
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\ttindex{None}
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\item zero of any numeric type, e.g., \code{0}, \code{0L}, \code{0.0}.
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\item any empty sequence, e.g., \code{''}, \code{()}, \code{[]}.
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\item any empty mapping, e.g., \code{\{\}}.
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\item instances of user-defined classes, if the class defines a
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\code{__nonzero__()} or \code{__len__()} method, when that
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method returns zero.
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\end{itemize}
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All other values are considered true --- so objects of many types are
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always true.
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\index{true}
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Operations and built-in functions that have a Boolean result always
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return \code{0} for false and \code{1} for true, unless otherwise
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stated. (Important exception: the Boolean operations \samp{or} and
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\samp{and} always return one of their operands.)
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\subsection{Boolean Operations}
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These are the Boolean operations, ordered by ascending priority:
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\indexii{Boolean}{operations}
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\begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
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\lineiii{\var{x} or \var{y}}{if \var{x} is false, then \var{y}, else \var{x}}{(1)}
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\hline
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\lineiii{\var{x} and \var{y}}{if \var{x} is false, then \var{x}, else \var{y}}{(1)}
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\hline
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\lineiii{not \var{x}}{if \var{x} is false, then \code{1}, else \code{0}}{(2)}
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\end{tableiii}
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\opindex{and}
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\opindex{or}
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\opindex{not}
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\noindent
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Notes:
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\begin{description}
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\item[(1)]
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These only evaluate their second argument if needed for their outcome.
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\item[(2)]
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\samp{not} has a lower priority than non-Boolean operators, so e.g.
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\code{not a == b} is interpreted as \code{not(a == b)}, and
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\code{a == not b} is a syntax error.
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\end{description}
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\subsection{Comparisons}
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Comparison operations are supported by all objects. They all have the
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same priority (which is higher than that of the Boolean operations).
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Comparisons can be chained arbitrarily, e.g. \code{x < y <= z} is
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equivalent to \code{x < y and y <= z}, except that \code{y} is
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evaluated only once (but in both cases \code{z} is not evaluated at
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all when \code{x < y} is found to be false).
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\indexii{chaining}{comparisons}
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This table summarizes the comparison operations:
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\begin{tableiii}{|c|l|c|}{code}{Operation}{Meaning}{Notes}
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\lineiii{<}{strictly less than}{}
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\lineiii{<=}{less than or equal}{}
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\lineiii{>}{strictly greater than}{}
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\lineiii{>=}{greater than or equal}{}
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\lineiii{==}{equal}{}
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\lineiii{<>}{not equal}{(1)}
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\lineiii{!=}{not equal}{(1)}
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\lineiii{is}{object identity}{}
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\lineiii{is not}{negated object identity}{}
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\end{tableiii}
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\indexii{operator}{comparison}
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\opindex{==} % XXX *All* others have funny characters < ! >
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\opindex{is}
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\opindex{is not}
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\noindent
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Notes:
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\begin{description}
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\item[(1)]
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\code{<>} and \code{!=} are alternate spellings for the same operator.
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(I couldn't choose between \ABC{} and \C{}! :-)
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\indexii{\ABC{}}{language}
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\indexii{\C{}}{language}
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\end{description}
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Objects of different types, except different numeric types, never
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compare equal; such objects are ordered consistently but arbitrarily
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(so that sorting a heterogeneous array yields a consistent result).
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Furthermore, some types (e.g., windows) support only a degenerate
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notion of comparison where any two objects of that type are unequal.
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Again, such objects are ordered arbitrarily but consistently.
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\indexii{types}{numeric}
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\indexii{objects}{comparing}
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(Implementation note: objects of different types except numbers are
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ordered by their type names; objects of the same types that don't
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support proper comparison are ordered by their address.)
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Two more operations with the same syntactic priority, \code{in} and
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\code{not in}, are supported only by sequence types (below).
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\opindex{in}
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\opindex{not in}
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\subsection{Numeric Types}
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There are four numeric types: \dfn{plain integers}, \dfn{long integers},
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\dfn{floating point numbers}, and \dfn{complex numbers}.
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Plain integers (also just called \dfn{integers})
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are implemented using \code{long} in \C{}, which gives them at least 32
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bits of precision. Long integers have unlimited precision. Floating
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point numbers are implemented using \code{double} in \C{}. All bets on
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their precision are off unless you happen to know the machine you are
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working with.
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\indexii{numeric}{types}
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\indexii{integer}{types}
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\indexii{integer}{type}
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\indexiii{long}{integer}{type}
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\indexii{floating point}{type}
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\indexii{complex number}{type}
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\indexii{\C{}}{language}
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Complex numbers have a real and imaginary part, which are both
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implemented using \code{double} in \C{}. To extract these parts from
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a complex number \code{z}, use \code{z.real} and \code{z.imag}.
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Numbers are created by numeric literals or as the result of built-in
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functions and operators. Unadorned integer literals (including hex
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and octal numbers) yield plain integers. Integer literals with an \samp{L}
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or \samp{l} suffix yield long integers
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(\samp{L} is preferred because \code{1l} looks too much like eleven!).
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Numeric literals containing a decimal point or an exponent sign yield
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floating point numbers. Appending \code{j} or \code{J} to a numeric
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literal yields a complex number.
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\indexii{numeric}{literals}
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\indexii{integer}{literals}
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\indexiii{long}{integer}{literals}
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\indexii{floating point}{literals}
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\indexii{complex number}{literals}
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\indexii{hexadecimal}{literals}
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\indexii{octal}{literals}
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Python fully supports mixed arithmetic: when a binary arithmetic
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operator has operands of different numeric types, the operand with the
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``smaller'' type is converted to that of the other, where plain
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integer is smaller than long integer is smaller than floating point is
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smaller than complex.
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Comparisons between numbers of mixed type use the same rule.%
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\footnote{As a consequence, the list \code{[1, 2]} is considered equal
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to \code{[1.0, 2.0]}, and similar for tuples.}
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The functions \code{int()}, \code{long()}, \code{float()},
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and \code{complex()} can be used
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to coerce numbers to a specific type.
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\index{arithmetic}
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\bifuncindex{int}
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\bifuncindex{long}
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\bifuncindex{float}
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\bifuncindex{complex}
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All numeric types support the following operations, sorted by
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ascending priority (operations in the same box have the same
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priority; all numeric operations have a higher priority than
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comparison operations):
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\begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
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\lineiii{\var{x} + \var{y}}{sum of \var{x} and \var{y}}{}
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\lineiii{\var{x} - \var{y}}{difference of \var{x} and \var{y}}{}
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\hline
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\lineiii{\var{x} * \var{y}}{product of \var{x} and \var{y}}{}
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\lineiii{\var{x} / \var{y}}{quotient of \var{x} and \var{y}}{(1)}
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\lineiii{\var{x} \%{} \var{y}}{remainder of \code{\var{x} / \var{y}}}{}
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\hline
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\lineiii{-\var{x}}{\var{x} negated}{}
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\lineiii{+\var{x}}{\var{x} unchanged}{}
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\hline
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\lineiii{abs(\var{x})}{absolute value or magnitude of \var{x}}{}
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\lineiii{int(\var{x})}{\var{x} converted to integer}{(2)}
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\lineiii{long(\var{x})}{\var{x} converted to long integer}{(2)}
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\lineiii{float(\var{x})}{\var{x} converted to floating point}{}
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\lineiii{complex(\var{re},\var{im})}{a complex number with real part \var{re}, imaginary part \var{im}. \var{im} defaults to zero.}{}
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\lineiii{divmod(\var{x}, \var{y})}{the pair \code{(\var{x} / \var{y}, \var{x} \%{} \var{y})}}{(3)}
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\lineiii{pow(\var{x}, \var{y})}{\var{x} to the power \var{y}}{}
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\lineiii{\var{x}**\var{y}}{\var{x} to the power \var{y}}{}
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\end{tableiii}
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\indexiii{operations on}{numeric}{types}
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\noindent
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Notes:
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\begin{description}
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\item[(1)]
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For (plain or long) integer division, the result is an integer.
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The result is always rounded towards minus infinity: 1/2 is 0,
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(-1)/2 is -1, 1/(-2) is -1, and (-1)/(-2) is 0.
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\indexii{integer}{division}
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\indexiii{long}{integer}{division}
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\item[(2)]
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Conversion from floating point to (long or plain) integer may round or
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truncate as in \C{}; see functions \code{floor()} and \code{ceil()} in
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module \code{math} for well-defined conversions.
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\bifuncindex{floor}
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\bifuncindex{ceil}
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\indexii{numeric}{conversions}
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\stmodindex{math}
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\indexii{\C{}}{language}
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\item[(3)]
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See the section on built-in functions for an exact definition.
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\end{description}
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% XXXJH exceptions: overflow (when? what operations?) zerodivision
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\subsubsection{Bit-string Operations on Integer Types}
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\nodename{Bit-string Operations}
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Plain and long integer types support additional operations that make
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sense only for bit-strings. Negative numbers are treated as their 2's
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complement value (for long integers, this assumes a sufficiently large
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number of bits that no overflow occurs during the operation).
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The priorities of the binary bit-wise operations are all lower than
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the numeric operations and higher than the comparisons; the unary
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operation \samp{~} has the same priority as the other unary numeric
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operations (\samp{+} and \samp{-}).
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This table lists the bit-string operations sorted in ascending
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priority (operations in the same box have the same priority):
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\begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
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\lineiii{\var{x} | \var{y}}{bitwise \dfn{or} of \var{x} and \var{y}}{}
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\hline
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\lineiii{\var{x} \^{} \var{y}}{bitwise \dfn{exclusive or} of \var{x} and \var{y}}{}
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\hline
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\lineiii{\var{x} \&{} \var{y}}{bitwise \dfn{and} of \var{x} and \var{y}}{}
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\hline
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\lineiii{\var{x} << \var{n}}{\var{x} shifted left by \var{n} bits}{(1), (2)}
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\lineiii{\var{x} >> \var{n}}{\var{x} shifted right by \var{n} bits}{(1), (3)}
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\hline
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\hline
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\lineiii{\~\var{x}}{the bits of \var{x} inverted}{}
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\end{tableiii}
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\indexiii{operations on}{integer}{types}
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\indexii{bit-string}{operations}
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\indexii{shifting}{operations}
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\indexii{masking}{operations}
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\noindent
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Notes:
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\begin{description}
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\item[(1)] Negative shift counts are illegal.
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\item[(2)] A left shift by \var{n} bits is equivalent to
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multiplication by \code{pow(2, \var{n})} without overflow check.
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\item[(3)] A right shift by \var{n} bits is equivalent to
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division by \code{pow(2, \var{n})} without overflow check.
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\end{description}
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\subsection{Sequence Types}
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There are three sequence types: strings, lists and tuples.
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Strings literals are written in single or double quotes:
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\code{'xyzzy'}, \code{"frobozz"}. See Chapter 2 of the Python
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Reference Manual for more about string literals. Lists are
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constructed with square brackets, separating items with commas:
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\code{[a, b, c]}. Tuples are constructed by the comma operator (not
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within square brackets), with or without enclosing parentheses, but an
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empty tuple must have the enclosing parentheses, e.g.,
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\code{a, b, c} or \code{()}. A single item tuple must have a trailing
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comma, e.g., \code{(d,)}.
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\indexii{sequence}{types}
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\indexii{string}{type}
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\indexii{tuple}{type}
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\indexii{list}{type}
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Sequence types support the following operations. The \samp{in} and
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\samp{not in} operations have the same priorities as the comparison
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operations. The \samp{+} and \samp{*} operations have the same
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priority as the corresponding numeric operations.\footnote{They must
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have since the parser can't tell the type of the operands.}
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This table lists the sequence operations sorted in ascending priority
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(operations in the same box have the same priority). In the table,
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\var{s} and \var{t} are sequences of the same type; \var{n}, \var{i}
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and \var{j} are integers:
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\begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
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\lineiii{\var{x} in \var{s}}{\code{1} if an item of \var{s} is equal to \var{x}, else \code{0}}{}
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\lineiii{\var{x} not in \var{s}}{\code{0} if an item of \var{s} is
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equal to \var{x}, else \code{1}}{}
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\hline
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\lineiii{\var{s} + \var{t}}{the concatenation of \var{s} and \var{t}}{}
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\hline
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\lineiii{\var{s} * \var{n}{\rm ,} \var{n} * \var{s}}{\var{n} copies of \var{s} concatenated}{}
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\hline
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\lineiii{\var{s}[\var{i}]}{\var{i}'th item of \var{s}, origin 0}{(1)}
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\lineiii{\var{s}[\var{i}:\var{j}]}{slice of \var{s} from \var{i} to \var{j}}{(1), (2)}
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\hline
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\lineiii{len(\var{s})}{length of \var{s}}{}
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\lineiii{min(\var{s})}{smallest item of \var{s}}{}
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\lineiii{max(\var{s})}{largest item of \var{s}}{}
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\end{tableiii}
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\indexiii{operations on}{sequence}{types}
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\bifuncindex{len}
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\bifuncindex{min}
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\bifuncindex{max}
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\indexii{concatenation}{operation}
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\indexii{repetition}{operation}
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\indexii{subscript}{operation}
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\indexii{slice}{operation}
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\opindex{in}
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\opindex{not in}
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\noindent
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Notes:
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\begin{description}
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\item[(1)] If \var{i} or \var{j} is negative, the index is relative to
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the end of the string, i.e., \code{len(\var{s}) + \var{i}} or
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\code{len(\var{s}) + \var{j}} is substituted. But note that \code{-0} is
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still \code{0}.
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\item[(2)] The slice of \var{s} from \var{i} to \var{j} is defined as
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the sequence of items with index \var{k} such that \code{\var{i} <=
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\var{k} < \var{j}}. If \var{i} or \var{j} is greater than
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\code{len(\var{s})}, use \code{len(\var{s})}. If \var{i} is omitted,
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use \code{0}. If \var{j} is omitted, use \code{len(\var{s})}. If
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\var{i} is greater than or equal to \var{j}, the slice is empty.
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\end{description}
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\subsubsection{More String Operations}
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String objects have one unique built-in operation: the \code{\%}
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operator (modulo) with a string left argument interprets this string
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as a C sprintf format string to be applied to the right argument, and
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returns the string resulting from this formatting operation.
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The right argument should be a tuple with one item for each argument
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required by the format string; if the string requires a single
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argument, the right argument may also be a single non-tuple object.%
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\footnote{A tuple object in this case should be a singleton.}
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The following format characters are understood:
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\%, c, s, i, d, u, o, x, X, e, E, f, g, G.
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Width and precision may be a * to specify that an integer argument
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specifies the actual width or precision. The flag characters -, +,
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blank, \# and 0 are understood. The size specifiers h, l or L may be
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present but are ignored. The \code{\%s} conversion takes any Python
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object and converts it to a string using \code{str()} before
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formatting it. The ANSI features \code{\%p} and \code{\%n}
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are not supported. Since Python strings have an explicit length,
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\code{\%s} conversions don't assume that \code{'\e0'} is the end of
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the string.
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For safety reasons, floating point precisions are clipped to 50;
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\code{\%f} conversions for numbers whose absolute value is over 1e25
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are replaced by \code{\%g} conversions.%
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\footnote{These numbers are fairly arbitrary. They are intended to
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avoid printing endless strings of meaningless digits without hampering
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correct use and without having to know the exact precision of floating
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point values on a particular machine.}
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All other errors raise exceptions.
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If the right argument is a dictionary (or any kind of mapping), then
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the formats in the string must have a parenthesized key into that
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dictionary inserted immediately after the \code{\%} character, and
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each format formats the corresponding entry from the mapping. E.g.
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\bcode\begin{verbatim}
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>>> count = 2
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>>> language = 'Python'
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>>> print '%(language)s has %(count)03d quote types.' % vars()
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Python has 002 quote types.
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>>>
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\end{verbatim}\ecode
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In this case no * specifiers may occur in a format (since they
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require a sequential parameter list).
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Additional string operations are defined in standard module
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\code{string} and in built-in module \code{regex}.
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\index{string}
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\index{regex}
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\subsubsection{Mutable Sequence Types}
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List objects support additional operations that allow in-place
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modification of the object.
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These operations would be supported by other mutable sequence types
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(when added to the language) as well.
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Strings and tuples are immutable sequence types and such objects cannot
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be modified once created.
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The following operations are defined on mutable sequence types (where
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\var{x} is an arbitrary object):
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\indexiii{mutable}{sequence}{types}
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\indexii{list}{type}
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\begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
|
|
\lineiii{\var{s}[\var{i}] = \var{x}}
|
|
{item \var{i} of \var{s} is replaced by \var{x}}{}
|
|
\lineiii{\var{s}[\var{i}:\var{j}] = \var{t}}
|
|
{slice of \var{s} from \var{i} to \var{j} is replaced by \var{t}}{}
|
|
\lineiii{del \var{s}[\var{i}:\var{j}]}
|
|
{same as \code{\var{s}[\var{i}:\var{j}] = []}}{}
|
|
\lineiii{\var{s}.append(\var{x})}
|
|
{same as \code{\var{s}[len(\var{s}):len(\var{s})] = [\var{x}]}}{}
|
|
\lineiii{\var{s}.count(\var{x})}
|
|
{return number of \var{i}'s for which \code{\var{s}[\var{i}] == \var{x}}}{}
|
|
\lineiii{\var{s}.index(\var{x})}
|
|
{return smallest \var{i} such that \code{\var{s}[\var{i}] == \var{x}}}{(1)}
|
|
\lineiii{\var{s}.insert(\var{i}, \var{x})}
|
|
{same as \code{\var{s}[\var{i}:\var{i}] = [\var{x}]}
|
|
if \code{\var{i} >= 0}}{}
|
|
\lineiii{\var{s}.remove(\var{x})}
|
|
{same as \code{del \var{s}[\var{s}.index(\var{x})]}}{(1)}
|
|
\lineiii{\var{s}.reverse()}
|
|
{reverses the items of \var{s} in place}{(3)}
|
|
\lineiii{\var{s}.sort()}
|
|
{sort the items of \var{s} in place}{(2), (3)}
|
|
\end{tableiii}
|
|
\indexiv{operations on}{mutable}{sequence}{types}
|
|
\indexiii{operations on}{sequence}{types}
|
|
\indexiii{operations on}{list}{type}
|
|
\indexii{subscript}{assignment}
|
|
\indexii{slice}{assignment}
|
|
\stindex{del}
|
|
\renewcommand{\indexsubitem}{(list method)}
|
|
\ttindex{append}
|
|
\ttindex{count}
|
|
\ttindex{index}
|
|
\ttindex{insert}
|
|
\ttindex{remove}
|
|
\ttindex{reverse}
|
|
\ttindex{sort}
|
|
|
|
\noindent
|
|
Notes:
|
|
\begin{description}
|
|
\item[(1)] Raises an exception when \var{x} is not found in \var{s}.
|
|
|
|
\item[(2)] The \code{sort()} method takes an optional argument
|
|
specifying a comparison function of two arguments (list items) which
|
|
should return \code{-1}, \code{0} or \code{1} depending on whether the
|
|
first argument is considered smaller than, equal to, or larger than the
|
|
second argument. Note that this slows the sorting process down
|
|
considerably; e.g. to sort a list in reverse order it is much faster
|
|
to use calls to \code{sort()} and \code{reverse()} than to use
|
|
\code{sort()} with a comparison function that reverses the ordering of
|
|
the elements.
|
|
|
|
\item[(3)] The \code{sort()} and \code{reverse()} methods modify the
|
|
list in place for economy of space when sorting or reversing a large
|
|
list. They don't return the sorted or reversed list to remind you of
|
|
this side effect.
|
|
|
|
\end{description}
|
|
|
|
\subsection{Mapping Types}
|
|
|
|
A \dfn{mapping} object maps values of one type (the key type) to
|
|
arbitrary objects. Mappings are mutable objects. There is currently
|
|
only one standard mapping type, the \dfn{dictionary}. A dictionary's keys are
|
|
almost arbitrary values. The only types of values not acceptable as
|
|
keys are values containing lists or dictionaries or other mutable
|
|
types that are compared by value rather than by object identity.
|
|
Numeric types used for keys obey the normal rules for numeric
|
|
comparison: if two numbers compare equal (e.g. 1 and 1.0) then they
|
|
can be used interchangeably to index the same dictionary entry.
|
|
|
|
\indexii{mapping}{types}
|
|
\indexii{dictionary}{type}
|
|
|
|
Dictionaries are created by placing a comma-separated list of
|
|
\code{\var{key}:\,\var{value}} pairs within braces, for example:
|
|
\code{\{'jack':\,4098, 'sjoerd':\,4127\}} or
|
|
\code{\{4098:\,'jack', 4127:\,'sjoerd'\}}.
|
|
|
|
The following operations are defined on mappings (where \var{a} is a
|
|
mapping, \var{k} is a key and \var{x} is an arbitrary object):
|
|
|
|
\begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
|
|
\lineiii{len(\var{a})}{the number of items in \var{a}}{}
|
|
\lineiii{\var{a}[\var{k}]}{the item of \var{a} with key \var{k}}{(1)}
|
|
\lineiii{\var{a}[\var{k}] = \var{x}}{set \code{\var{a}[\var{k}]} to \var{x}}{}
|
|
\lineiii{del \var{a}[\var{k}]}{remove \code{\var{a}[\var{k}]} from \var{a}}{(1)}
|
|
\lineiii{\var{a}.clear()}{remove all items from \code{a}}{}
|
|
\lineiii{\var{a}.copy()}{a (shallow) copy of \code{a}}{}
|
|
\lineiii{\var{a}.has_key(\var{k})}{\code{1} if \var{a} has a key \var{k}, else \code{0}}{}
|
|
\lineiii{\var{a}.items()}{a copy of \var{a}'s list of (key, item) pairs}{(2)}
|
|
\lineiii{\var{a}.keys()}{a copy of \var{a}'s list of keys}{(2)}
|
|
\lineiii{\var{a}.update(b)}{\code{for k, v in b.items(): a[k] = v}}{(3)}
|
|
\lineiii{\var{a}.values()}{a copy of \var{a}'s list of values}{(2)}
|
|
\lineiii{\var{a}.get(k, f)}{the item of \var{a} with key \var{k}}{(4)}
|
|
\end{tableiii}
|
|
\indexiii{operations on}{mapping}{types}
|
|
\indexiii{operations on}{dictionary}{type}
|
|
\stindex{del}
|
|
\bifuncindex{len}
|
|
\renewcommand{\indexsubitem}{(dictionary method)}
|
|
\ttindex{keys}
|
|
\ttindex{has_key}
|
|
|
|
\noindent
|
|
Notes:
|
|
\begin{description}
|
|
\item[(1)] Raises an exception if \var{k} is not in the map.
|
|
|
|
\item[(2)] Keys and values are listed in random order.
|
|
|
|
\item[(3)] \code{b} must be of the same type as \code{a}.
|
|
|
|
\item[(4)] Never raises an exception if \var{k} is not in the map,
|
|
instead it returns \var{f}. \var{f} is optional, when not provided
|
|
and \var{k} is not in the map, \code{None} is returned.
|
|
\end{description}
|
|
|
|
\subsection{Other Built-in Types}
|
|
|
|
The interpreter supports several other kinds of objects.
|
|
Most of these support only one or two operations.
|
|
|
|
\subsubsection{Modules}
|
|
|
|
The only special operation on a module is attribute access:
|
|
\code{\var{m}.\var{name}}, where \var{m} is a module and \var{name} accesses
|
|
a name defined in \var{m}'s symbol table. Module attributes can be
|
|
assigned to. (Note that the \code{import} statement is not, strictly
|
|
spoken, an operation on a module object; \code{import \var{foo}} does not
|
|
require a module object named \var{foo} to exist, rather it requires
|
|
an (external) \emph{definition} for a module named \var{foo}
|
|
somewhere.)
|
|
|
|
A special member of every module is \code{__dict__}.
|
|
This is the dictionary containing the module's symbol table.
|
|
Modifying this dictionary will actually change the module's symbol
|
|
table, but direct assignment to the \code{__dict__} attribute is not
|
|
possible (i.e., you can write \code{\var{m}.__dict__['a'] = 1}, which
|
|
defines \code{\var{m}.a} to be \code{1}, but you can't write \code{\var{m}.__dict__ = \{\}}.
|
|
|
|
Modules are written like this: \code{<module 'sys'>}.
|
|
|
|
\subsubsection{Classes and Class Instances}
|
|
\nodename{Classes and Instances}
|
|
|
|
(See Chapters 3 and 7 of the Python Reference Manual for these.)
|
|
|
|
\subsubsection{Functions}
|
|
|
|
Function objects are created by function definitions. The only
|
|
operation on a function object is to call it:
|
|
\code{\var{func}(\var{argument-list})}.
|
|
|
|
There are really two flavors of function objects: built-in functions
|
|
and user-defined functions. Both support the same operation (to call
|
|
the function), but the implementation is different, hence the
|
|
different object types.
|
|
|
|
The implementation adds two special read-only attributes:
|
|
\code{\var{f}.func_code} is a function's \dfn{code object} (see below) and
|
|
\code{\var{f}.func_globals} is the dictionary used as the function's
|
|
global name space (this is the same as \code{\var{m}.__dict__} where
|
|
\var{m} is the module in which the function \var{f} was defined).
|
|
|
|
\subsubsection{Methods}
|
|
\obindex{method}
|
|
|
|
Methods are functions that are called using the attribute notation.
|
|
There are two flavors: built-in methods (such as \code{append()} on
|
|
lists) and class instance methods. Built-in methods are described
|
|
with the types that support them.
|
|
|
|
The implementation adds two special read-only attributes to class
|
|
instance methods: \code{\var{m}.im_self} is the object whose method this
|
|
is, and \code{\var{m}.im_func} is the function implementing the method.
|
|
Calling \code{\var{m}(\var{arg-1}, \var{arg-2}, {\rm \ldots},
|
|
\var{arg-n})} is completely equivalent to calling
|
|
\code{\var{m}.im_func(\var{m}.im_self, \var{arg-1}, \var{arg-2}, {\rm
|
|
\ldots}, \var{arg-n})}.
|
|
|
|
(See the Python Reference Manual for more info.)
|
|
|
|
\subsubsection{Code Objects}
|
|
\obindex{code}
|
|
|
|
Code objects are used by the implementation to represent
|
|
``pseudo-compiled'' executable Python code such as a function body.
|
|
They differ from function objects because they don't contain a
|
|
reference to their global execution environment. Code objects are
|
|
returned by the built-in \code{compile()} function and can be
|
|
extracted from function objects through their \code{func_code}
|
|
attribute.
|
|
\bifuncindex{compile}
|
|
\ttindex{func_code}
|
|
|
|
A code object can be executed or evaluated by passing it (instead of a
|
|
source string) to the \code{exec} statement or the built-in
|
|
\code{eval()} function.
|
|
\stindex{exec}
|
|
\bifuncindex{eval}
|
|
|
|
(See the Python Reference Manual for more info.)
|
|
|
|
\subsubsection{Type Objects}
|
|
|
|
Type objects represent the various object types. An object's type is
|
|
accessed by the built-in function \code{type()}. There are no special
|
|
operations on types. The standard module \code{types} defines names
|
|
for all standard built-in types.
|
|
\bifuncindex{type}
|
|
\stmodindex{types}
|
|
|
|
Types are written like this: \code{<type 'int'>}.
|
|
|
|
\subsubsection{The Null Object}
|
|
|
|
This object is returned by functions that don't explicitly return a
|
|
value. It supports no special operations. There is exactly one null
|
|
object, named \code{None} (a built-in name).
|
|
|
|
It is written as \code{None}.
|
|
|
|
\subsubsection{File Objects}
|
|
|
|
File objects are implemented using \C{}'s \code{stdio} package and can be
|
|
created with the built-in function \code{open()} described under
|
|
Built-in Functions below. They are also returned by some other
|
|
built-in functions and methods, e.g.\ \code{posix.popen()} and
|
|
\code{posix.fdopen()} and the \code{makefile()} method of socket
|
|
objects.
|
|
\bifuncindex{open}
|
|
|
|
When a file operation fails for an I/O-related reason, the exception
|
|
\code{IOError} is raised. This includes situations where the
|
|
operation is not defined for some reason, like \code{seek()} on a tty
|
|
device or writing a file opened for reading.
|
|
|
|
Files have the following methods:
|
|
|
|
|
|
\renewcommand{\indexsubitem}{(file method)}
|
|
|
|
\begin{funcdesc}{close}{}
|
|
Close the file. A closed file cannot be read or written anymore.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{flush}{}
|
|
Flush the internal buffer, like \code{stdio}'s \code{fflush()}.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{isatty}{}
|
|
Return \code{1} if the file is connected to a tty(-like) device, else
|
|
\code{0}.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{fileno}{}
|
|
Return the integer ``file descriptor'' that is used by the underlying
|
|
implementation to request I/O operations from the operating system.
|
|
This can be useful for other, lower level interfaces that use file
|
|
descriptors, e.g. module \code{fcntl} or \code{os.read} and friends.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{read}{\optional{size}}
|
|
Read at most \var{size} bytes from the file (less if the read hits
|
|
\EOF{} or no more data is immediately available on a pipe, tty or
|
|
similar device). If the \var{size} argument is negative or omitted,
|
|
read all data until \EOF{} is reached. The bytes are returned as a string
|
|
object. An empty string is returned when \EOF{} is encountered
|
|
immediately. (For certain files, like ttys, it makes sense to
|
|
continue reading after an \EOF{} is hit.)
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{readline}{\optional{size}}
|
|
Read one entire line from the file. A trailing newline character is
|
|
kept in the string%
|
|
\footnote{The advantage of leaving the newline on is that an empty string
|
|
can be returned to mean \EOF{} without being ambiguous. Another
|
|
advantage is that (in cases where it might matter, e.g. if you
|
|
want to make an exact copy of a file while scanning its lines)
|
|
you can tell whether the last line of a file ended in a newline
|
|
or not (yes this happens!).}
|
|
(but may be absent when a file ends with an
|
|
incomplete line). If the \var{size} argument is present and
|
|
non-negative, it is a maximum byte count (including the trailing
|
|
newline) and an incomplete line may be returned.
|
|
An empty string is returned when \EOF{} is hit
|
|
immediately. Note: unlike \code{stdio}'s \code{fgets()}, the returned
|
|
string contains null characters (\code{'\e 0'}) if they occurred in the
|
|
input.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{readlines}{\optional{sizehint}}
|
|
Read until \EOF{} using \code{readline()} and return a list containing
|
|
the lines thus read. If the optional \var{bufferhint} argument is
|
|
present, instead of reading up to \EOF{}, whole lines totalling
|
|
approximately \var{sizehint} bytes are read.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{seek}{offset\, whence}
|
|
Set the file's current position, like \code{stdio}'s \code{fseek()}.
|
|
The \var{whence} argument is optional and defaults to \code{0}
|
|
(absolute file positioning); other values are \code{1} (seek
|
|
relative to the current position) and \code{2} (seek relative to the
|
|
file's end). There is no return value.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{tell}{}
|
|
Return the file's current position, like \code{stdio}'s \code{ftell()}.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{truncate}{\optional{size}}
|
|
Truncate the file's size. If the optional size argument present, the
|
|
file is truncated to (at most) that size. The size defaults to the
|
|
current position. Availability of this function depends on the
|
|
operating system version (e.g., not all \UNIX{} versions support this
|
|
operation).
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{write}{str}
|
|
Write a string to the file. There is no return value. Note: due to
|
|
buffering, the string may not actually show up in the file until
|
|
the \code{flush()} or \code{close()} method is called.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{writelines}{list}
|
|
Write a list of strings to the file. There is no return value.
|
|
(The name is intended to match \code{readlines}; \code{writelines}
|
|
does not add line separators.)
|
|
\end{funcdesc}
|
|
|
|
Classes that are trying to simulate a file object should also have a
|
|
writable \code{softspace} attribute, which should be initialized to
|
|
zero. (\code{softspace} is used by the \code{print} statement.) This
|
|
will be automatic for classes implemented in Python; types implemented
|
|
in C will have to provide a writable \code{softspace} attribute.
|
|
|
|
\subsubsection{Internal Objects}
|
|
|
|
(See the Python Reference Manual for these.)
|
|
|
|
\subsection{Special Attributes}
|
|
|
|
The implementation adds a few special read-only attributes to several
|
|
object types, where they are relevant:
|
|
|
|
\begin{itemize}
|
|
|
|
\item
|
|
\code{\var{x}.__dict__} is a dictionary of some sort used to store an
|
|
object's (writable) attributes;
|
|
|
|
\item
|
|
\code{\var{x}.__methods__} lists the methods of many built-in object types,
|
|
e.g., \code{[].__methods__} yields
|
|
\code{['append', 'count', 'index', 'insert', 'remove', 'reverse', 'sort']};
|
|
|
|
\item
|
|
\code{\var{x}.__members__} lists data attributes;
|
|
|
|
\item
|
|
\code{\var{x}.__class__} is the class to which a class instance belongs;
|
|
|
|
\item
|
|
\code{\var{x}.__bases__} is the tuple of base classes of a class object.
|
|
|
|
\end{itemize}
|