os
module provides dozens of functions for interacting with the operating system:
>>> import os
>>> os.getcwd() # Return the current working directory
'C:\\Python36'
>>> os.chdir('/server/accesslogs') # Change current working directory
>>> os.system('mkdir today') # Run the command mkdir in the system shell
0
Be sure to use the
import
os
style instead of
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'
glob
module provides a function for making file lists from directory wildcard searches:
>>> import glob
>>> glob.glob('*.py')
['primes.py', 'random.py', 'quote.py']
Common utility scripts often need to process command line arguments. These arguments are stored in the
sys
module’s
argv
attribute as a list. For instance the following output results from running
python
demo.py
one
two
three
at the command line:
>>> import sys
>>> print(sys.argv)
['demo.py', 'one', 'two', 'three']
getopt
module processes
sys.argv
using the conventions of the Unix
getopt()
function. More powerful and flexible command line processing is provided by the
argparse
模块。
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()
.
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'
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
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
0.17970987693706186
>>> random.randrange(6) # random integer chosen from range(6)
4
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
The SciPy project < https://scipy.org > has many other modules for numerical computations.
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://tycho.usno.navy.mil/cgi-bin/timer.pl') as response:
... for line in response:
... line = line.decode('utf-8') # Decoding the binary data to text.
... if 'EST' in line or 'EDT' in line: # look for Eastern Time
... print(line)
<BR>Nov. 25, 09:43:32 PM EST
>>> 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.)
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
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
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
In contrast to
timeit
’s fine level of granularity, the
profile
and
pstats
modules provide tools for identifying time critical sections in larger blocks of code.
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.
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
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
Python has a “batteries included” philosophy. This is best seen through the sophisticated and robust capabilities of its larger packages. For example:
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.
email
package is a library for managing email messages, including MIME and other
RFC 2822
-based message documents. Unlike
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.
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.
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.
gettext
,
locale
，和
codecs
package.
11. Brief Tour of the Standard Library — Part II