mirror of
https://github.com/python/cpython.git
synced 2024-11-23 18:04:37 +08:00
#7499: Review of Library FAQ by Florent Xicluna.
This commit is contained in:
Georg Brandl
parent
62eaaf62ff
commit
9e4ff75325
@ -38,7 +38,7 @@ There are (at least) three kinds of modules in Python:
|
||||
type::
|
||||
|
||||
import sys
|
||||
print sys.builtin_module_names
|
||||
print(sys.builtin_module_names)
|
||||
|
||||
|
||||
How do I make a Python script executable on Unix?
|
||||
@ -187,8 +187,11 @@ How do I get a single keypress at a time?
|
||||
-----------------------------------------
|
||||
|
||||
For Unix variants: There are several solutions. It's straightforward to do this
|
||||
using curses, but curses is a fairly large module to learn. Here's a solution
|
||||
without curses::
|
||||
using curses, but curses is a fairly large module to learn.
|
||||
|
||||
.. XXX this doesn't work out of the box, some IO expert needs to check why
|
||||
|
||||
Here's a solution without curses::
|
||||
|
||||
import termios, fcntl, sys, os
|
||||
fd = sys.stdin.fileno()
|
||||
@ -202,23 +205,24 @@ without curses::
|
||||
fcntl.fcntl(fd, fcntl.F_SETFL, oldflags | os.O_NONBLOCK)
|
||||
|
||||
try:
|
||||
while 1:
|
||||
while True:
|
||||
try:
|
||||
c = sys.stdin.read(1)
|
||||
print "Got character", `c`
|
||||
except IOError: pass
|
||||
print("Got character", repr(c))
|
||||
except IOError:
|
||||
pass
|
||||
finally:
|
||||
termios.tcsetattr(fd, termios.TCSAFLUSH, oldterm)
|
||||
fcntl.fcntl(fd, fcntl.F_SETFL, oldflags)
|
||||
|
||||
You need the :mod:`termios` and the :mod:`fcntl` module for any of this to work,
|
||||
and I've only tried it on Linux, though it should work elsewhere. In this code,
|
||||
characters are read and printed one at a time.
|
||||
You need the :mod:`termios` and the :mod:`fcntl` module for any of this to
|
||||
work, and I've only tried it on Linux, though it should work elsewhere. In
|
||||
this code, characters are read and printed one at a time.
|
||||
|
||||
:func:`termios.tcsetattr` turns off stdin's echoing and disables canonical mode.
|
||||
:func:`fcntl.fnctl` is used to obtain stdin's file descriptor flags and modify
|
||||
them for non-blocking mode. Since reading stdin when it is empty results in an
|
||||
:exc:`IOError`, this error is caught and ignored.
|
||||
:func:`termios.tcsetattr` turns off stdin's echoing and disables canonical
|
||||
mode. :func:`fcntl.fnctl` is used to obtain stdin's file descriptor flags
|
||||
and modify them for non-blocking mode. Since reading stdin when it is empty
|
||||
results in an :exc:`IOError`, this error is caught and ignored.
|
||||
|
||||
|
||||
Threads
|
||||
@ -247,13 +251,13 @@ all the threads to finish::
|
||||
import threading, time
|
||||
|
||||
def thread_task(name, n):
|
||||
for i in range(n): print name, i
|
||||
for i in range(n): print(name, i)
|
||||
|
||||
for i in range(10):
|
||||
T = threading.Thread(target=thread_task, args=(str(i), i))
|
||||
T.start()
|
||||
|
||||
time.sleep(10) # <----------------------------!
|
||||
time.sleep(10) # <---------------------------!
|
||||
|
||||
But now (on many platforms) the threads don't run in parallel, but appear to run
|
||||
sequentially, one at a time! The reason is that the OS thread scheduler doesn't
|
||||
@ -262,8 +266,8 @@ start a new thread until the previous thread is blocked.
|
||||
A simple fix is to add a tiny sleep to the start of the run function::
|
||||
|
||||
def thread_task(name, n):
|
||||
time.sleep(0.001) # <---------------------!
|
||||
for i in range(n): print name, i
|
||||
time.sleep(0.001) # <--------------------!
|
||||
for i in range(n): print(name, i)
|
||||
|
||||
for i in range(10):
|
||||
T = threading.Thread(target=thread_task, args=(str(i), i))
|
||||
@ -289,28 +293,28 @@ once.
|
||||
|
||||
Here's a trivial example::
|
||||
|
||||
import threading, Queue, time
|
||||
import threading, queue, time
|
||||
|
||||
# The worker thread gets jobs off the queue. When the queue is empty, it
|
||||
# assumes there will be no more work and exits.
|
||||
# (Realistically workers will run until terminated.)
|
||||
def worker ():
|
||||
print 'Running worker'
|
||||
print('Running worker')
|
||||
time.sleep(0.1)
|
||||
while True:
|
||||
try:
|
||||
arg = q.get(block=False)
|
||||
except Queue.Empty:
|
||||
print 'Worker', threading.currentThread(),
|
||||
print 'queue empty'
|
||||
except queue.Empty:
|
||||
print('Worker', threading.currentThread(), end=' ')
|
||||
print('queue empty')
|
||||
break
|
||||
else:
|
||||
print 'Worker', threading.currentThread(),
|
||||
print 'running with argument', arg
|
||||
print('Worker', threading.currentThread(), end=' ')
|
||||
print('running with argument', arg)
|
||||
time.sleep(0.5)
|
||||
|
||||
# Create queue
|
||||
q = Queue.Queue()
|
||||
q = queue.Queue()
|
||||
|
||||
# Start a pool of 5 workers
|
||||
for i in range(5):
|
||||
@ -322,10 +326,10 @@ Here's a trivial example::
|
||||
q.put(i)
|
||||
|
||||
# Give threads time to run
|
||||
print 'Main thread sleeping'
|
||||
print('Main thread sleeping')
|
||||
time.sleep(5)
|
||||
|
||||
When run, this will produce the following output:
|
||||
When run, this will produce the following output::
|
||||
|
||||
Running worker
|
||||
Running worker
|
||||
@ -333,12 +337,12 @@ When run, this will produce the following output:
|
||||
Running worker
|
||||
Running worker
|
||||
Main thread sleeping
|
||||
Worker <Thread(worker 1, started)> running with argument 0
|
||||
Worker <Thread(worker 2, started)> running with argument 1
|
||||
Worker <Thread(worker 3, started)> running with argument 2
|
||||
Worker <Thread(worker 4, started)> running with argument 3
|
||||
Worker <Thread(worker 5, started)> running with argument 4
|
||||
Worker <Thread(worker 1, started)> running with argument 5
|
||||
Worker <Thread(worker 1, started 130283832797456)> running with argument 0
|
||||
Worker <Thread(worker 2, started 130283824404752)> running with argument 1
|
||||
Worker <Thread(worker 3, started 130283816012048)> running with argument 2
|
||||
Worker <Thread(worker 4, started 130283807619344)> running with argument 3
|
||||
Worker <Thread(worker 5, started 130283799226640)> running with argument 4
|
||||
Worker <Thread(worker 1, started 130283832797456)> running with argument 5
|
||||
...
|
||||
|
||||
Consult the module's documentation for more details; the ``Queue`` class
|
||||
@ -351,7 +355,7 @@ What kinds of global value mutation are thread-safe?
|
||||
A global interpreter lock (GIL) is used internally to ensure that only one
|
||||
thread runs in the Python VM at a time. In general, Python offers to switch
|
||||
among threads only between bytecode instructions; how frequently it switches can
|
||||
be set via :func:`sys.setcheckinterval`. Each bytecode instruction and
|
||||
be set via :func:`sys.setswitchinterval`. Each bytecode instruction and
|
||||
therefore all the C implementation code reached from each instruction is
|
||||
therefore atomic from the point of view of a Python program.
|
||||
|
||||
@ -443,7 +447,7 @@ How do I delete a file? (And other file questions...)
|
||||
-----------------------------------------------------
|
||||
|
||||
Use ``os.remove(filename)`` or ``os.unlink(filename)``; for documentation, see
|
||||
the :mod:`os` module. The two functions are identical; :func:`unlink` is simply
|
||||
the :mod:`os` module. The two functions are identical; :func:`~os.unlink` is simply
|
||||
the name of the Unix system call for this function.
|
||||
|
||||
To remove a directory, use :func:`os.rmdir`; use :func:`os.mkdir` to create one.
|
||||
@ -505,81 +509,83 @@ to read n bytes from a pipe p created with :func:`os.popen`, you need to use
|
||||
``p.read(n)``.
|
||||
|
||||
|
||||
How do I run a subprocess with pipes connected to both input and output?
|
||||
------------------------------------------------------------------------
|
||||
.. XXX update to use subprocess. See the :ref:`subprocess-replacements` section.
|
||||
|
||||
.. XXX update to use subprocess
|
||||
How do I run a subprocess with pipes connected to both input and output?
|
||||
------------------------------------------------------------------------
|
||||
|
||||
Use the :mod:`popen2` module. For example::
|
||||
Use the :mod:`popen2` module. For example::
|
||||
|
||||
import popen2
|
||||
fromchild, tochild = popen2.popen2("command")
|
||||
tochild.write("input\n")
|
||||
tochild.flush()
|
||||
output = fromchild.readline()
|
||||
import popen2
|
||||
fromchild, tochild = popen2.popen2("command")
|
||||
tochild.write("input\n")
|
||||
tochild.flush()
|
||||
output = fromchild.readline()
|
||||
|
||||
Warning: in general it is unwise to do this because you can easily cause a
|
||||
deadlock where your process is blocked waiting for output from the child while
|
||||
the child is blocked waiting for input from you. This can be caused because the
|
||||
parent expects the child to output more text than it does, or it can be caused
|
||||
by data being stuck in stdio buffers due to lack of flushing. The Python parent
|
||||
can of course explicitly flush the data it sends to the child before it reads
|
||||
any output, but if the child is a naive C program it may have been written to
|
||||
never explicitly flush its output, even if it is interactive, since flushing is
|
||||
normally automatic.
|
||||
Warning: in general it is unwise to do this because you can easily cause a
|
||||
deadlock where your process is blocked waiting for output from the child
|
||||
while the child is blocked waiting for input from you. This can be caused
|
||||
because the parent expects the child to output more text than it does, or it
|
||||
can be caused by data being stuck in stdio buffers due to lack of flushing.
|
||||
The Python parent can of course explicitly flush the data it sends to the
|
||||
child before it reads any output, but if the child is a naive C program it
|
||||
may have been written to never explicitly flush its output, even if it is
|
||||
interactive, since flushing is normally automatic.
|
||||
|
||||
Note that a deadlock is also possible if you use :func:`popen3` to read stdout
|
||||
and stderr. If one of the two is too large for the internal buffer (increasing
|
||||
the buffer size does not help) and you ``read()`` the other one first, there is
|
||||
a deadlock, too.
|
||||
Note that a deadlock is also possible if you use :func:`popen3` to read
|
||||
stdout and stderr. If one of the two is too large for the internal buffer
|
||||
(increasing the buffer size does not help) and you ``read()`` the other one
|
||||
first, there is a deadlock, too.
|
||||
|
||||
Note on a bug in popen2: unless your program calls ``wait()`` or ``waitpid()``,
|
||||
finished child processes are never removed, and eventually calls to popen2 will
|
||||
fail because of a limit on the number of child processes. Calling
|
||||
:func:`os.waitpid` with the :data:`os.WNOHANG` option can prevent this; a good
|
||||
place to insert such a call would be before calling ``popen2`` again.
|
||||
Note on a bug in popen2: unless your program calls ``wait()`` or
|
||||
``waitpid()``, finished child processes are never removed, and eventually
|
||||
calls to popen2 will fail because of a limit on the number of child
|
||||
processes. Calling :func:`os.waitpid` with the :data:`os.WNOHANG` option can
|
||||
prevent this; a good place to insert such a call would be before calling
|
||||
``popen2`` again.
|
||||
|
||||
In many cases, all you really need is to run some data through a command and get
|
||||
the result back. Unless the amount of data is very large, the easiest way to do
|
||||
this is to write it to a temporary file and run the command with that temporary
|
||||
file as input. The standard module :mod:`tempfile` exports a ``mktemp()``
|
||||
function to generate unique temporary file names. ::
|
||||
In many cases, all you really need is to run some data through a command and
|
||||
get the result back. Unless the amount of data is very large, the easiest
|
||||
way to do this is to write it to a temporary file and run the command with
|
||||
that temporary file as input. The standard module :mod:`tempfile` exports a
|
||||
``mktemp()`` function to generate unique temporary file names. ::
|
||||
|
||||
import tempfile
|
||||
import os
|
||||
import tempfile
|
||||
import os
|
||||
|
||||
class Popen3:
|
||||
"""
|
||||
This is a deadlock-safe version of popen that returns
|
||||
an object with errorlevel, out (a string) and err (a string).
|
||||
(capturestderr may not work under windows.)
|
||||
Example: print Popen3('grep spam','\n\nhere spam\n\n').out
|
||||
"""
|
||||
def __init__(self,command,input=None,capturestderr=None):
|
||||
outfile=tempfile.mktemp()
|
||||
command="( %s ) > %s" % (command,outfile)
|
||||
if input:
|
||||
infile=tempfile.mktemp()
|
||||
open(infile,"w").write(input)
|
||||
command=command+" <"+infile
|
||||
if capturestderr:
|
||||
errfile=tempfile.mktemp()
|
||||
command=command+" 2>"+errfile
|
||||
self.errorlevel=os.system(command) >> 8
|
||||
self.out=open(outfile,"r").read()
|
||||
os.remove(outfile)
|
||||
if input:
|
||||
os.remove(infile)
|
||||
if capturestderr:
|
||||
self.err=open(errfile,"r").read()
|
||||
os.remove(errfile)
|
||||
class Popen3:
|
||||
"""
|
||||
This is a deadlock-safe version of popen that returns
|
||||
an object with errorlevel, out (a string) and err (a string).
|
||||
(capturestderr may not work under windows.)
|
||||
Example: print(Popen3('grep spam','\n\nhere spam\n\n').out)
|
||||
"""
|
||||
def __init__(self,command,input=None,capturestderr=None):
|
||||
outfile=tempfile.mktemp()
|
||||
command="( %s ) > %s" % (command,outfile)
|
||||
if input:
|
||||
infile=tempfile.mktemp()
|
||||
open(infile,"w").write(input)
|
||||
command=command+" <"+infile
|
||||
if capturestderr:
|
||||
errfile=tempfile.mktemp()
|
||||
command=command+" 2>"+errfile
|
||||
self.errorlevel=os.system(command) >> 8
|
||||
self.out=open(outfile,"r").read()
|
||||
os.remove(outfile)
|
||||
if input:
|
||||
os.remove(infile)
|
||||
if capturestderr:
|
||||
self.err=open(errfile,"r").read()
|
||||
os.remove(errfile)
|
||||
|
||||
Note that many interactive programs (e.g. vi) don't work well with pipes
|
||||
substituted for standard input and output. You will have to use pseudo ttys
|
||||
("ptys") instead of pipes. Or you can use a Python interface to Don Libes'
|
||||
"expect" library. A Python extension that interfaces to expect is called "expy"
|
||||
and available from http://expectpy.sourceforge.net. A pure Python solution that
|
||||
works like expect is `pexpect <http://pypi.python.org/pypi/pexpect/>`_.
|
||||
Note that many interactive programs (e.g. vi) don't work well with pipes
|
||||
substituted for standard input and output. You will have to use pseudo ttys
|
||||
("ptys") instead of pipes. Or you can use a Python interface to Don Libes'
|
||||
"expect" library. A Python extension that interfaces to expect is called
|
||||
"expy" and available from http://expectpy.sourceforge.net. A pure Python
|
||||
solution that works like expect is `pexpect
|
||||
<http://pypi.python.org/pypi/pexpect/>`_.
|
||||
|
||||
|
||||
How do I access the serial (RS232) port?
|
||||
@ -601,7 +607,7 @@ Python file objects are a high-level layer of abstraction on top of C streams,
|
||||
which in turn are a medium-level layer of abstraction on top of (among other
|
||||
things) low-level C file descriptors.
|
||||
|
||||
For most file objects you create in Python via the builtin ``file`` constructor,
|
||||
For most file objects you create in Python via the builtin ``open`` constructor,
|
||||
``f.close()`` marks the Python file object as being closed from Python's point
|
||||
of view, and also arranges to close the underlying C stream. This also happens
|
||||
automatically in f's destructor, when f becomes garbage.
|
||||
@ -645,41 +651,29 @@ How can I mimic CGI form submission (METHOD=POST)?
|
||||
I would like to retrieve web pages that are the result of POSTing a form. Is
|
||||
there existing code that would let me do this easily?
|
||||
|
||||
Yes. Here's a simple example that uses httplib::
|
||||
Yes. Here's a simple example that uses urllib.request::
|
||||
|
||||
#!/usr/local/bin/python
|
||||
|
||||
import httplib, sys, time
|
||||
import urllib.request
|
||||
|
||||
### build the query string
|
||||
qs = "First=Josephine&MI=Q&Last=Public"
|
||||
|
||||
### connect and send the server a path
|
||||
httpobj = httplib.HTTP('www.some-server.out-there', 80)
|
||||
httpobj.putrequest('POST', '/cgi-bin/some-cgi-script')
|
||||
### now generate the rest of the HTTP headers...
|
||||
httpobj.putheader('Accept', '*/*')
|
||||
httpobj.putheader('Connection', 'Keep-Alive')
|
||||
httpobj.putheader('Content-type', 'application/x-www-form-urlencoded')
|
||||
httpobj.putheader('Content-length', '%d' % len(qs))
|
||||
httpobj.endheaders()
|
||||
httpobj.send(qs)
|
||||
### find out what the server said in response...
|
||||
reply, msg, hdrs = httpobj.getreply()
|
||||
if reply != 200:
|
||||
sys.stdout.write(httpobj.getfile().read())
|
||||
req = urllib.request.urlopen('http://www.some-server.out-there'
|
||||
'/cgi-bin/some-cgi-script', data=qs)
|
||||
msg, hdrs = req.read(), req.info()
|
||||
|
||||
Note that in general for URL-encoded POST operations, query strings must be
|
||||
quoted by using :func:`urllib.quote`. For example to send name="Guy Steele,
|
||||
quoted by using :func:`urllib.parse.urlencode`. For example to send name="Guy Steele,
|
||||
Jr."::
|
||||
|
||||
>>> from urllib import quote
|
||||
>>> x = quote("Guy Steele, Jr.")
|
||||
>>> x
|
||||
'Guy%20Steele,%20Jr.'
|
||||
>>> query_string = "name="+x
|
||||
>>> query_string
|
||||
'name=Guy%20Steele,%20Jr.'
|
||||
>>> import urllib.parse
|
||||
>>> urllib.parse.urlencode({'name': 'Guy Steele, Jr.'})
|
||||
'name=Guy+Steele%2C+Jr.'
|
||||
|
||||
.. seealso:: :ref:`urllib-howto` for extensive examples.
|
||||
|
||||
|
||||
What module should I use to help with generating HTML?
|
||||
@ -712,9 +706,9 @@ work on any host that supports an SMTP listener. ::
|
||||
|
||||
import sys, smtplib
|
||||
|
||||
fromaddr = raw_input("From: ")
|
||||
toaddrs = raw_input("To: ").split(',')
|
||||
print "Enter message, end with ^D:"
|
||||
fromaddr = input("From: ")
|
||||
toaddrs = input("To: ").split(',')
|
||||
print("Enter message, end with ^D:")
|
||||
msg = ''
|
||||
while True:
|
||||
line = sys.stdin.readline()
|
||||
@ -732,17 +726,17 @@ varies between systems; sometimes it is ``/usr/lib/sendmail``, sometime
|
||||
``/usr/sbin/sendmail``. The sendmail manual page will help you out. Here's
|
||||
some sample code::
|
||||
|
||||
SENDMAIL = "/usr/sbin/sendmail" # sendmail location
|
||||
SENDMAIL = "/usr/sbin/sendmail" # sendmail location
|
||||
import os
|
||||
p = os.popen("%s -t -i" % SENDMAIL, "w")
|
||||
p.write("To: receiver@example.com\n")
|
||||
p.write("Subject: test\n")
|
||||
p.write("\n") # blank line separating headers from body
|
||||
p.write("\n") # blank line separating headers from body
|
||||
p.write("Some text\n")
|
||||
p.write("some more text\n")
|
||||
sts = p.close()
|
||||
if sts != 0:
|
||||
print "Sendmail exit status", sts
|
||||
print("Sendmail exit status", sts)
|
||||
|
||||
|
||||
How do I avoid blocking in the connect() method of a socket?
|
||||
@ -759,7 +753,7 @@ have to check what's returned on your system.
|
||||
|
||||
You can use the ``connect_ex()`` method to avoid creating an exception. It will
|
||||
just return the errno value. To poll, you can call ``connect_ex()`` again later
|
||||
-- 0 or ``errno.EISCONN`` indicate that you're connected -- or you can pass this
|
||||
-- ``0`` or ``errno.EISCONN`` indicate that you're connected -- or you can pass this
|
||||
socket to select to check if it's writable.
|
||||
|
||||
|
||||
@ -798,22 +792,16 @@ than a third of a second. This often beats doing something more complex and
|
||||
general such as using gdbm with pickle/shelve.
|
||||
|
||||
|
||||
Why is cPickle so slow?
|
||||
-----------------------
|
||||
|
||||
.. XXX update this, default protocol is 2/3
|
||||
|
||||
The default format used by the pickle module is a slow one that results in
|
||||
readable pickles. Making it the default, but it would break backward
|
||||
compatibility::
|
||||
|
||||
largeString = 'z' * (100 * 1024)
|
||||
myPickle = cPickle.dumps(largeString, protocol=1)
|
||||
|
||||
|
||||
If my program crashes with a bsddb (or anydbm) database open, it gets corrupted. How come?
|
||||
------------------------------------------------------------------------------------------
|
||||
|
||||
.. XXX move this FAQ entry elsewhere?
|
||||
|
||||
.. note::
|
||||
|
||||
The bsddb module is now available as a standalone package `pybsddb
|
||||
<http://www.jcea.es/programacion/pybsddb.htm>`_.
|
||||
|
||||
Databases opened for write access with the bsddb module (and often by the anydbm
|
||||
module, since it will preferentially use bsddb) must explicitly be closed using
|
||||
the ``.close()`` method of the database. The underlying library caches database
|
||||
@ -827,6 +815,13 @@ encounter an exception the next time the file is opened.
|
||||
I tried to open Berkeley DB file, but bsddb produces bsddb.error: (22, 'Invalid argument'). Help! How can I restore my data?
|
||||
----------------------------------------------------------------------------------------------------------------------------
|
||||
|
||||
.. XXX move this FAQ entry elsewhere?
|
||||
|
||||
.. note::
|
||||
|
||||
The bsddb module is now available as a standalone package `pybsddb
|
||||
<http://www.jcea.es/programacion/pybsddb.htm>`_.
|
||||
|
||||
Don't panic! Your data is probably intact. The most frequent cause for the error
|
||||
is that you tried to open an earlier Berkeley DB file with a later version of
|
||||
the Berkeley DB library.
|
||||
|
||||
@ -1,3 +1,5 @@
|
||||
.. _urllib-howto:
|
||||
|
||||
***********************************************************
|
||||
HOWTO Fetch Internet Resources Using The urllib Package
|
||||
***********************************************************
|
||||
|
||||
Loading…
Reference in New Issue
Block a user