python_doc_md/Intermediate/faq/library.md

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# Library and Extension FAQ

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## General Library Questions

### How do I find a module or application to perform task X?

Check `the Library Reference <library-index>`{.interpreted-text role="ref"} to see if there\'s a relevant standard library module. (Eventually you\'ll learn what\'s in the standard library and will be able to skip this step.)

For third-party packages, search the [Python Package Index](https://pypi.org) or try [Google](https://www.google.com) or another web search engine. Searching for \"Python\" plus a keyword or two for your topic of interest will usually find something helpful.

### Where is the math.py (socket.py, regex.py, etc.) source file?

If you can\'t find a source file for a module it may be a built-in or dynamically loaded module implemented in C, C++ or other compiled language. In this case you may not have the source file or it may be something like `mathmodule.c`{.interpreted-text role="file"}, somewhere in a C source directory (not on the Python Path).

There are (at least) three kinds of modules in Python:

1)  modules written in Python (.py);

2)  modules written in C and dynamically loaded (.dll, .pyd, .so, .sl, etc);

3)  modules written in C and linked with the interpreter; to get a list of these, type:

        import sys
        print(sys.builtin_module_names)

### How do I make a Python script executable on Unix?

You need to do two things: the script file\'s mode must be executable and the first line must begin with `#!` followed by the path of the Python interpreter.

The first is done by executing `chmod +x scriptfile` or perhaps `chmod 755 scriptfile`.

The second can be done in a number of ways. The most straightforward way is to write :

    #!/usr/local/bin/python

as the very first line of your file, using the pathname for where the Python interpreter is installed on your platform.

If you would like the script to be independent of where the Python interpreter lives, you can use the `env`{.interpreted-text role="program"} program. Almost all Unix variants support the following, assuming the Python interpreter is in a directory on the user\'s `PATH`{.interpreted-text role="envvar"}:

    #!/usr/bin/env python

*Don\'t* do this for CGI scripts. The `PATH`{.interpreted-text role="envvar"} variable for CGI scripts is often very minimal, so you need to use the actual absolute pathname of the interpreter.

Occasionally, a user\'s environment is so full that the `/usr/bin/env`{.interpreted-text role="program"} program fails; or there\'s no env program at all. In that case, you can try the following hack (due to Alex Rezinsky):

``` sh
#! /bin/sh
""":"
exec python $0 ${1+"$@"}
"""
```

The minor disadvantage is that this defines the script\'s \_\_doc\_\_ string. However, you can fix that by adding :

    __doc__ = """...Whatever..."""

### Is there a curses/termcap package for Python?

For Unix variants: The standard Python source distribution comes with a curses module in the `Modules`{.interpreted-text role="source"} subdirectory, though it\'s not compiled by default. (Note that this is not available in the Windows distribution \-- there is no curses module for Windows.)

The `curses`{.interpreted-text role="mod"} module supports basic curses features as well as many additional functions from ncurses and SYSV curses such as colour, alternative character set support, pads, and mouse support. This means the module isn\'t compatible with operating systems that only have BSD curses, but there don\'t seem to be any currently maintained OSes that fall into this category.

### Is there an equivalent to C\'s onexit() in Python?

The `atexit`{.interpreted-text role="mod"} module provides a register function that is similar to C\'s `!onexit`{.interpreted-text role="c:func"}.

### Why don\'t my signal handlers work?

The most common problem is that the signal handler is declared with the wrong argument list. It is called as :

    handler(signum, frame)

so it should be declared with two parameters:

    def handler(signum, frame):
        ...

## Common tasks

### How do I test a Python program or component?

Python comes with two testing frameworks. The `doctest`{.interpreted-text role="mod"} module finds examples in the docstrings for a module and runs them, comparing the output with the expected output given in the docstring.

The `unittest`{.interpreted-text role="mod"} module is a fancier testing framework modelled on Java and Smalltalk testing frameworks.

To make testing easier, you should use good modular design in your program. Your program should have almost all functionality encapsulated in either functions or class methods \-- and this sometimes has the surprising and delightful effect of making the program run faster (because local variable accesses are faster than global accesses). Furthermore the program should avoid depending on mutating global variables, since this makes testing much more difficult to do.

The \"global main logic\" of your program may be as simple as :

    if __name__ == "__main__":
        main_logic()

at the bottom of the main module of your program.

Once your program is organized as a tractable collection of function and class behaviours, you should write test functions that exercise the behaviours. A test suite that automates a sequence of tests can be associated with each module. This sounds like a lot of work, but since Python is so terse and flexible it\'s surprisingly easy. You can make coding much more pleasant and fun by writing your test functions in parallel with the \"production code\", since this makes it easy to find bugs and even design flaws earlier.

\"Support modules\" that are not intended to be the main module of a program may include a self-test of the module. :

    if __name__ == "__main__":
        self_test()

Even programs that interact with complex external interfaces may be tested when the external interfaces are unavailable by using \"fake\" interfaces implemented in Python.

### How do I create documentation from doc strings?

The `pydoc`{.interpreted-text role="mod"} module can create HTML from the doc strings in your Python source code. An alternative for creating API documentation purely from docstrings is [epydoc](https://epydoc.sourceforge.net/). [Sphinx](https://www.sphinx-doc.org) can also include docstring content.

### 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:
>
> import termios, fcntl, sys, os fd = sys.stdin.fileno()
>
> oldterm = termios.tcgetattr(fd) newattr = termios.tcgetattr(fd) newattr\[3\] = newattr\[3\] & \~termios.ICANON & \~termios.ECHO termios.tcsetattr(fd, termios.TCSANOW, newattr)
>
> oldflags = fcntl.fcntl(fd, fcntl.F_GETFL) fcntl.fcntl(fd, fcntl.F_SETFL, oldflags \| os.O_NONBLOCK)
>
> try:
>
> :   
>
>     while True:
>
>     :   
>
>         try:
>
>         :   c = sys.stdin.read(1) print(\"Got character\", repr(c))
>
>         except OSError:
>
>         :   pass
>
> finally:
>
> :   termios.tcsetattr(fd, termios.TCSAFLUSH, oldterm) fcntl.fcntl(fd, fcntl.F_SETFL, oldflags)
>
> You need the `termios`{.interpreted-text role="mod"} and the `fcntl`{.interpreted-text role="mod"} 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.
>
> `termios.tcsetattr`{.interpreted-text role="func"} turns off stdin\'s echoing and disables canonical mode. `fcntl.fnctl`{.interpreted-text role="func"} 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 `OSError`{.interpreted-text role="exc"}, this error is caught and ignored.
>
> ::: versionchanged
> 3.3 *sys.stdin.read* used to raise `IOError`{.interpreted-text role="exc"}. Starting from Python 3.3 `IOError`{.interpreted-text role="exc"} is alias for `OSError`{.interpreted-text role="exc"}.
> :::

## Threads

### How do I program using threads?

Be sure to use the `threading`{.interpreted-text role="mod"} module and not the `_thread`{.interpreted-text role="mod"} module. The `threading`{.interpreted-text role="mod"} module builds convenient abstractions on top of the low-level primitives provided by the `_thread`{.interpreted-text role="mod"} module.

### None of my threads seem to run: why?

As soon as the main thread exits, all threads are killed. Your main thread is running too quickly, giving the threads no time to do any work.

A simple fix is to add a sleep to the end of the program that\'s long enough for 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(10):
        T = threading.Thread(target=thread_task, args=(str(i), i))
        T.start()

    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 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)

    for i in range(10):
        T = threading.Thread(target=thread_task, args=(str(i), i))
        T.start()

    time.sleep(10)

Instead of trying to guess a good delay value for `time.sleep`{.interpreted-text role="func"}, it\'s better to use some kind of semaphore mechanism. One idea is to use the `queue`{.interpreted-text role="mod"} module to create a queue object, let each thread append a token to the queue when it finishes, and let the main thread read as many tokens from the queue as there are threads.

### How do I parcel out work among a bunch of worker threads?

The easiest way is to use the `concurrent.futures`{.interpreted-text role="mod"} module, especially the `~concurrent.futures.ThreadPoolExecutor`{.interpreted-text role="mod"} class.

Or, if you want fine control over the dispatching algorithm, you can write your own logic manually. Use the `queue`{.interpreted-text role="mod"} module to create a queue containing a list of jobs. The `~queue.Queue`{.interpreted-text role="class"} class maintains a list of objects and has a `.put(obj)` method that adds items to the queue and a `.get()` method to return them. The class will take care of the locking necessary to ensure that each job is handed out exactly once.

Here\'s a trivial example:

    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')
        time.sleep(0.1)
        while True:
            try:
                arg = q.get(block=False)
            except queue.Empty:
                print('Worker', threading.current_thread(), end=' ')
                print('queue empty')
                break
            else:
                print('Worker', threading.current_thread(), end=' ')
                print('running with argument', arg)
                time.sleep(0.5)

    # Create queue
    q = queue.Queue()

    # Start a pool of 5 workers
    for i in range(5):
        t = threading.Thread(target=worker, name='worker %i' % (i+1))
        t.start()

    # Begin adding work to the queue
    for i in range(50):
        q.put(i)

    # Give threads time to run
    print('Main thread sleeping')
    time.sleep(5)

When run, this will produce the following output:

``` none
Running worker
Running worker
Running worker
Running worker
Running worker
Main thread sleeping
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.Queue`{.interpreted-text role="class"} class provides a featureful interface.

### What kinds of global value mutation are thread-safe?

A `global interpreter lock`{.interpreted-text role="term"} (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 `sys.setswitchinterval`{.interpreted-text role="func"}. 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.

In theory, this means an exact accounting requires an exact understanding of the PVM bytecode implementation. In practice, it means that operations on shared variables of built-in data types (ints, lists, dicts, etc) that \"look atomic\" really are.

For example, the following operations are all atomic (L, L1, L2 are lists, D, D1, D2 are dicts, x, y are objects, i, j are ints):

    L.append(x)
    L1.extend(L2)
    x = L[i]
    x = L.pop()
    L1[i:j] = L2
    L.sort()
    x = y
    x.field = y
    D[x] = y
    D1.update(D2)
    D.keys()

These aren\'t:

    i = i+1
    L.append(L[-1])
    L[i] = L[j]
    D[x] = D[x] + 1

Operations that replace other objects may invoke those other objects\' `~object.__del__`{.interpreted-text role="meth"} method when their reference count reaches zero, and that can affect things. This is especially true for the mass updates to dictionaries and lists. When in doubt, use a mutex!

### Can\'t we get rid of the Global Interpreter Lock?

The `global interpreter lock`{.interpreted-text role="term"} (GIL) is often seen as a hindrance to Python\'s deployment on high-end multiprocessor server machines, because a multi-threaded Python program effectively only uses one CPU, due to the insistence that (almost) all Python code can only run while the GIL is held.

With the approval of `703`{.interpreted-text role="pep"} work is now underway to remove the GIL from the CPython implementation of Python. Initially it will be implemented as an optional compiler flag when building the interpreter, and so separate builds will be available with and without the GIL. Long-term, the hope is to settle on a single build, once the performance implications of removing the GIL are fully understood. Python 3.13 is likely to be the first release containing this work, although it may not be completely functional in this release.

The current work to remove the GIL is based on a [fork of Python 3.9 with the GIL removed](https://github.com/colesbury/nogil) by Sam Gross. Prior to that, in the days of Python 1.5, Greg Stein actually implemented a comprehensive patch set (the \"free threading\" patches) that removed the GIL and replaced it with fine-grained locking. Adam Olsen did a similar experiment in his [python-safethread](https://code.google.com/archive/p/python-safethread) project. Unfortunately, both of these earlier experiments exhibited a sharp drop in single-thread performance (at least 30% slower), due to the amount of fine-grained locking necessary to compensate for the removal of the GIL. The Python 3.9 fork is the first attempt at removing the GIL with an acceptable performance impact.

The presence of the GIL in current Python releases doesn\'t mean that you can\'t make good use of Python on multi-CPU machines! You just have to be creative with dividing the work up between multiple *processes* rather than multiple *threads*. The `~concurrent.futures.ProcessPoolExecutor`{.interpreted-text role="class"} class in the new `concurrent.futures`{.interpreted-text role="mod"} module provides an easy way of doing so; the `multiprocessing`{.interpreted-text role="mod"} module provides a lower-level API in case you want more control over dispatching of tasks.

Judicious use of C extensions will also help; if you use a C extension to perform a time-consuming task, the extension can release the GIL while the thread of execution is in the C code and allow other threads to get some work done. Some standard library modules such as `zlib`{.interpreted-text role="mod"} and `hashlib`{.interpreted-text role="mod"} already do this.

An alternative approach to reducing the impact of the GIL is to make the GIL a per-interpreter-state lock rather than truly global. This was `first implemented in Python 3.12 <whatsnew312-pep684>`{.interpreted-text role="ref"} and is available in the C API. A Python interface to it is expected in Python 3.13. The main limitation to it at the moment is likely to be 3rd party extension modules, since these must be written with multiple interpreters in mind in order to be usable, so many older extension modules will not be usable.

## Input and Output

### How do I delete a file? (And other file questions\...)

Use `os.remove(filename)` or `os.unlink(filename)`; for documentation, see the `os`{.interpreted-text role="mod"} module. The two functions are identical; `~os.unlink`{.interpreted-text role="func"} is simply the name of the Unix system call for this function.

To remove a directory, use `os.rmdir`{.interpreted-text role="func"}; use `os.mkdir`{.interpreted-text role="func"} to create one. `os.makedirs(path)` will create any intermediate directories in `path` that don\'t exist. `os.removedirs(path)` will remove intermediate directories as long as they\'re empty; if you want to delete an entire directory tree and its contents, use `shutil.rmtree`{.interpreted-text role="func"}.

To rename a file, use `os.rename(old_path, new_path)`.

To truncate a file, open it using `f = open(filename, "rb+")`, and use `f.truncate(offset)`; offset defaults to the current seek position. There\'s also `os.ftruncate(fd, offset)` for files opened with `os.open`{.interpreted-text role="func"}, where *fd* is the file descriptor (a small integer).

The `shutil`{.interpreted-text role="mod"} module also contains a number of functions to work on files including `~shutil.copyfile`{.interpreted-text role="func"}, `~shutil.copytree`{.interpreted-text role="func"}, and `~shutil.rmtree`{.interpreted-text role="func"}.

### How do I copy a file?

The `shutil`{.interpreted-text role="mod"} module contains a `~shutil.copyfile`{.interpreted-text role="func"} function. Note that on Windows NTFS volumes, it does not copy [alternate data streams](https://en.wikipedia.org/wiki/NTFS#Alternate_data_stream_(ADS)) nor [resource forks](https://en.wikipedia.org/wiki/Resource_fork) on macOS HFS+ volumes, though both are now rarely used. It also doesn\'t copy file permissions and metadata, though using `shutil.copy2`{.interpreted-text role="func"} instead will preserve most (though not all) of it.

### How do I read (or write) binary data?

To read or write complex binary data formats, it\'s best to use the `struct`{.interpreted-text role="mod"} module. It allows you to take a string containing binary data (usually numbers) and convert it to Python objects; and vice versa.

For example, the following code reads two 2-byte integers and one 4-byte integer in big-endian format from a file:

    import struct

    with open(filename, "rb") as f:
        s = f.read(8)
        x, y, z = struct.unpack(">hhl", s)

The \'\>\' in the format string forces big-endian data; the letter \'h\' reads one \"short integer\" (2 bytes), and \'l\' reads one \"long integer\" (4 bytes) from the string.

For data that is more regular (e.g. a homogeneous list of ints or floats), you can also use the `array`{.interpreted-text role="mod"} module.

:::: note
::: title
Note
:::

To read and write binary data, it is mandatory to open the file in binary mode (here, passing `"rb"` to `open`{.interpreted-text role="func"}). If you use `"r"` instead (the default), the file will be open in text mode and `f.read()` will return `str`{.interpreted-text role="class"} objects rather than `bytes`{.interpreted-text role="class"} objects.
::::

### I can\'t seem to use os.read() on a pipe created with os.popen(); why?

`os.read`{.interpreted-text role="func"} is a low-level function which takes a file descriptor, a small integer representing the opened file. `os.popen`{.interpreted-text role="func"} creates a high-level file object, the same type returned by the built-in `open`{.interpreted-text role="func"} function. Thus, to read *n* bytes from a pipe *p* created with `os.popen`{.interpreted-text role="func"}, you need to use `p.read(n)`.

### How do I access the serial (RS232) port?

For Win32, OSX, Linux, BSD, Jython, IronPython:

> `pyserial`{.interpreted-text role="pypi"}

For Unix, see a Usenet post by Mitch Chapman:

> <https://groups.google.com/groups?selm=34A04430.CF9@ohioee.com>

### Why doesn\'t closing sys.stdout (stdin, stderr) really close it?

Python `file objects <file object>`{.interpreted-text role="term"} are a high-level layer of abstraction on low-level C file descriptors.

For most file objects you create in Python via the built-in `open`{.interpreted-text role="func"} function, `f.close()` marks the Python file object as being closed from Python\'s point of view, and also arranges to close the underlying C file descriptor. This also happens automatically in `f`\'s destructor, when `f` becomes garbage.

But stdin, stdout and stderr are treated specially by Python, because of the special status also given to them by C. Running `sys.stdout.close()` marks the Python-level file object as being closed, but does *not* close the associated C file descriptor.

To close the underlying C file descriptor for one of these three, you should first be sure that\'s what you really want to do (e.g., you may confuse extension modules trying to do I/O). If it is, use `os.close`{.interpreted-text role="func"}:

    os.close(stdin.fileno())
    os.close(stdout.fileno())
    os.close(stderr.fileno())

Or you can use the numeric constants 0, 1 and 2, respectively.

## Network/Internet Programming

### What WWW tools are there for Python?

See the chapters titled `internet`{.interpreted-text role="ref"} and `netdata`{.interpreted-text role="ref"} in the Library Reference Manual. Python has many modules that will help you build server-side and client-side web systems.

A summary of available frameworks is maintained by Paul Boddie at [https://wiki.python.org/moin/WebProgramming\\](https://wiki.python.org/moin/WebProgramming\) .

### What module should I use to help with generating HTML?

You can find a collection of useful links on the [Web Programming wiki page](https://wiki.python.org/moin/WebProgramming).

### How do I send mail from a Python script?

Use the standard library module `smtplib`{.interpreted-text role="mod"}.

Here\'s a very simple interactive mail sender that uses it. This method will work on any host that supports an SMTP listener. :

    import sys, smtplib

    fromaddr = input("From: ")
    toaddrs  = input("To: ").split(',')
    print("Enter message, end with ^D:")
    msg = ''
    while True:
        line = sys.stdin.readline()
        if not line:
            break
        msg += line

    # The actual mail send
    server = smtplib.SMTP('localhost')
    server.sendmail(fromaddr, toaddrs, msg)
    server.quit()

A Unix-only alternative uses sendmail. The location of the sendmail program varies between systems; sometimes it is `/usr/lib/sendmail`, sometimes `/usr/sbin/sendmail`. The sendmail manual page will help you out. Here\'s some sample code:

    import os

    SENDMAIL = "/usr/sbin/sendmail"  # sendmail location
    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("Some text\n")
    p.write("some more text\n")
    sts = p.close()
    if sts != 0:
        print("Sendmail exit status", sts)

### How do I avoid blocking in the connect() method of a socket?

The `select`{.interpreted-text role="mod"} module is commonly used to help with asynchronous I/O on sockets.

To prevent the TCP connect from blocking, you can set the socket to non-blocking mode. Then when you do the `~socket.socket.connect`{.interpreted-text role="meth"}, you will either connect immediately (unlikely) or get an exception that contains the error number as `.errno`. `errno.EINPROGRESS` indicates that the connection is in progress, but hasn\'t finished yet. Different OSes will return different values, so you\'re going to have to check what\'s returned on your system.

You can use the `~socket.socket.connect_ex`{.interpreted-text role="meth"} method to avoid creating an exception. It will just return the errno value. To poll, you can call `~socket.socket.connect_ex`{.interpreted-text role="meth"} again later \-- `0` or `errno.EISCONN` indicate that you\'re connected \-- or you can pass this socket to `select.select`{.interpreted-text role="meth"} to check if it\'s writable.

:::: note
::: title
Note
:::

The `asyncio`{.interpreted-text role="mod"} module provides a general purpose single-threaded and concurrent asynchronous library, which can be used for writing non-blocking network code. The third-party [Twisted](https://twisted.org/) library is a popular and feature-rich alternative.
::::

## Databases

### Are there any interfaces to database packages in Python?

Yes.

Interfaces to disk-based hashes such as `DBM <dbm.ndbm>`{.interpreted-text role="mod"} and `GDBM
<dbm.gnu>`{.interpreted-text role="mod"} are also included with standard Python. There is also the `sqlite3`{.interpreted-text role="mod"} module, which provides a lightweight disk-based relational database.

Support for most relational databases is available. See the [DatabaseProgramming wiki page](https://wiki.python.org/moin/DatabaseProgramming) for details.

### How do you implement persistent objects in Python?

The `pickle`{.interpreted-text role="mod"} library module solves this in a very general way (though you still can\'t store things like open files, sockets or windows), and the `shelve`{.interpreted-text role="mod"} library module uses pickle and (g)dbm to create persistent mappings containing arbitrary Python objects.

## Mathematics and Numerics

### How do I generate random numbers in Python?

The standard module `random`{.interpreted-text role="mod"} implements a random number generator. Usage is simple:

    import random
    random.random()

This returns a random floating-point number in the range \[0, 1).

There are also many other specialized generators in this module, such as:

- `randrange(a, b)` chooses an integer in the range \[a, b).
- `uniform(a, b)` chooses a floating-point number in the range \[a, b).
- `normalvariate(mean, sdev)` samples the normal (Gaussian) distribution.

Some higher-level functions operate on sequences directly, such as:

- `choice(S)` chooses a random element from a given sequence.
- `shuffle(L)` shuffles a list in-place, i.e. permutes it randomly.

There\'s also a `Random` class you can instantiate to create independent multiple random number generators.