10. 标准库简介 ¶

10.1. 操作系统接口 ¶

The os module provides dozens of functions for interacting with the operating system:

>>> import os
>>> os.getcwd()      # Return the current working directory
'C:\\Python313'
>>> os.chdir('/server/accesslogs')   # Change current working directory
>>> os.system('mkdir today')   # Run the command mkdir in the system shell
0

确保使用 import os 样式而不是 from os import * . This will keep os.open() from shadowing the built-in open() function which operates much differently.

内置 dir() and help() functions are useful as interactive aids for working with large modules like os :

>>> import os
>>> dir(os)
<returns a list of all module functions>
>>> help(os)
<returns an extensive manual page created from the module's docstrings>

For daily file and directory management tasks, the shutil module provides a higher level interface that is easier to use:

>>> import shutil
>>> shutil.copyfile('data.db', 'archive.db')
'archive.db'
>>> shutil.move('/build/executables', 'installdir')
'installdir'

10.2. File Wildcards ¶

The glob module provides a function for making file lists from directory wildcard searches:

>>> import glob
>>> glob.glob('*.py')
['primes.py', 'random.py', 'quote.py']

10.3. 命令行自变量 ¶

Common utility scripts often need to process command line arguments. These arguments are stored in the sys 模块的 argv attribute as a list. For instance, let’s take the following demo.py 文件：

# File demo.py
import sys
print(sys.argv)

Here is the output from running python demo.py one two three at the command line:

['demo.py', 'one', 'two', 'three']

The argparse module provides a more sophisticated mechanism to process command line arguments. The following script extracts one or more filenames and an optional number of lines to be displayed:

import argparse
parser = argparse.ArgumentParser(
    prog='top',
    description='Show top lines from each file')
parser.add_argument('filenames', nargs='+')
parser.add_argument('-l', '--lines', type=int, default=10)
args = parser.parse_args()
print(args)

When run at the command line with python top.py --lines=5 alpha.txt beta.txt , the script sets args.lines to 5 and args.filenames to ['alpha.txt', 'beta.txt'] .

10.4. 错误输出重定向和程序终止 ¶

The sys module also has attributes for stdin , stdout ，和 stderr . The latter is useful for emitting warnings and error messages to make them visible even when stdout has been redirected:

>>> sys.stderr.write('Warning, log file not found starting a new one\n')
Warning, log file not found starting a new one

The most direct way to terminate a script is to use sys.exit() .

10.5. 字符串模式匹配 ¶

The re module provides regular expression tools for advanced string processing. For complex matching and manipulation, regular expressions offer succinct, optimized solutions:

>>> import re
>>> re.findall(r'\bf[a-z]*', 'which foot or hand fell fastest')
['foot', 'fell', 'fastest']
>>> re.sub(r'(\b[a-z]+) \1', r'\1', 'cat in the the hat')
'cat in the hat'

When only simple capabilities are needed, string methods are preferred because they are easier to read and debug:

>>> 'tea for too'.replace('too', 'two')
'tea for two'

10.6. 数学 ¶

The math module gives access to the underlying C library functions for floating-point math:

>>> import math
>>> math.cos(math.pi / 4)
0.70710678118654757
>>> math.log(1024, 2)
10.0

The random module provides tools for making random selections:

>>> import random
>>> random.choice(['apple', 'pear', 'banana'])
'apple'
>>> random.sample(range(100), 10)   # sampling without replacement
[30, 83, 16, 4, 8, 81, 41, 50, 18, 33]
>>> random.random()    # random float from the interval [0.0, 1.0)
0.17970987693706186
>>> random.randrange(6)    # random integer chosen from range(6)
4

The statistics module calculates basic statistical properties (the mean, median, variance, etc.) of numeric data:

>>> import statistics
>>> data = [2.75, 1.75, 1.25, 0.25, 0.5, 1.25, 3.5]
>>> statistics.mean(data)
1.6071428571428572
>>> statistics.median(data)
1.25
>>> statistics.variance(data)
1.3720238095238095

SciPy 工程 < https://scipy.org > has many other modules for numerical computations.

10.7. Internet Access ¶

There are a number of modules for accessing the internet and processing internet protocols. Two of the simplest are urllib.request for retrieving data from URLs and smtplib for sending mail:

>>> from urllib.request import urlopen
>>> with urlopen('http://worldtimeapi.org/api/timezone/etc/UTC.txt') as response:
...     for line in response:
...         line = line.decode()             # Convert bytes to a str
...         if line.startswith('datetime'):
...             print(line.rstrip())         # Remove trailing newline
...
datetime: 2022-01-01T01:36:47.689215+00:00
>>> import smtplib
>>> server = smtplib.SMTP('localhost')
>>> server.sendmail('soothsayer@example.org', 'jcaesar@example.org',
... """To: jcaesar@example.org
... From: soothsayer@example.org
...
... Beware the Ides of March.
... """)
>>> server.quit()

(Note that the second example needs a mailserver running on localhost.)

10.8. 日期和时间 ¶

The datetime module supplies classes for manipulating dates and times in both simple and complex ways. While date and time arithmetic is supported, the focus of the implementation is on efficient member extraction for output formatting and manipulation. The module also supports objects that are timezone aware.

>>> # dates are easily constructed and formatted
>>> from datetime import date
>>> now = date.today()
>>> now
datetime.date(2003, 12, 2)
>>> now.strftime("%m-%d-%y. %d %b %Y is a %A on the %d day of %B.")
'12-02-03. 02 Dec 2003 is a Tuesday on the 02 day of December.'
>>> # dates support calendar arithmetic
>>> birthday = date(1964, 7, 31)
>>> age = now - birthday
>>> age.days
14368

10.9. 数据压缩 ¶

Common data archiving and compression formats are directly supported by modules including: zlib , gzip , bz2 , lzma , zipfile and tarfile .

>>> import zlib
>>> s = b'witch which has which witches wrist watch'
>>> len(s)
41
>>> t = zlib.compress(s)
>>> len(t)
37
>>> zlib.decompress(t)
b'witch which has which witches wrist watch'
>>> zlib.crc32(s)
226805979

10.10. 性能度量 ¶

Some Python users develop a deep interest in knowing the relative performance of different approaches to the same problem. Python provides a measurement tool that answers those questions immediately.

For example, it may be tempting to use the tuple packing and unpacking feature instead of the traditional approach to swapping arguments. The timeit module quickly demonstrates a modest performance advantage:

>>> from timeit import Timer
>>> Timer('t=a; a=b; b=t', 'a=1; b=2').timeit()
0.57535828626024577
>>> Timer('a,b = b,a', 'a=1; b=2').timeit()
0.54962537085770791

相比 timeit 的细粒度级别， profile and pstats modules provide tools for identifying time critical sections in larger blocks of code.

10.11. 品质控制 ¶

One approach for developing high quality software is to write tests for each function as it is developed and to run those tests frequently during the development process.

The doctest module provides a tool for scanning a module and validating tests embedded in a program’s docstrings. Test construction is as simple as cutting-and-pasting a typical call along with its results into the docstring. This improves the documentation by providing the user with an example and it allows the doctest module to make sure the code remains true to the documentation:

def average(values):
    """Computes the arithmetic mean of a list of numbers.
    >>> print(average([20, 30, 70]))
    40.0
    """
    return sum(values) / len(values)
import doctest
doctest.testmod()   # automatically validate the embedded tests

The unittest module is not as effortless as the doctest module, but it allows a more comprehensive set of tests to be maintained in a separate file:

import unittest
class TestStatisticalFunctions(unittest.TestCase):
    def test_average(self):
        self.assertEqual(average([20, 30, 70]), 40.0)
        self.assertEqual(round(average([1, 5, 7]), 1), 4.3)
        with self.assertRaises(ZeroDivisionError):
            average([])
        with self.assertRaises(TypeError):
            average(20, 30, 70)
unittest.main()  # Calling from the command line invokes all tests

10.12. 包括电池 ¶

Python 拥有 "包括电池" 的理念。这最好透过其的更大包的完备和健壮能力得以体现。例如：

The xmlrpc.client and xmlrpc.server modules make implementing remote procedure calls into an almost trivial task. Despite the modules’ names, no direct knowledge or handling of XML is needed.
The email 包是用于管理电子邮件的库，包括 MIME 和其它 RFC 2822 基消息文档。不像 smtplib and poplib which actually send and receive messages, the email package has a complete toolset for building or decoding complex message structures (including attachments) and for implementing internet encoding and header protocols.
The json package provides robust support for parsing this popular data interchange format. The csv module supports direct reading and writing of files in Comma-Separated Value format, commonly supported by databases and spreadsheets. XML processing is supported by the xml.etree.ElementTree , xml.dom and xml.sax packages. Together, these modules and packages greatly simplify data interchange between Python applications and other tools.
The sqlite3 module is a wrapper for the SQLite database library, providing a persistent database that can be updated and accessed using slightly nonstandard SQL syntax.
Internationalization is supported by a number of modules including gettext , locale ，和 codecs 包。

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