Python Modules

Python comes with many libraries covering a large variety of functions. During this section we will look at some of them.

Recall that libraries are accessed using the import statement. You can see what object a library contains by using dir(<module>) in ipython, and looking at <module>.<object>.__doc__ string

In [1]: import math

In [2]: print math.exp.__doc__

Return e raised to the power of x.

os module

The os module provide a platform independent way to work with the operating system, make or remove files and directories.

In [1]: import os

In [2]: print os.getcwd()

In [3]: os.chdir('/home/jacob/baz')

In [4]: os.getcwd()
Out[4]: '/home/jacob/baz'

In [5]: os.remove('foo')

In [6]: os.chdir('/home/jacob')

In [7]: os.rmdir('baz')

csv module

The csv module provides readers and writers for comma separated value data.

In [1]: import csv

In [2]: f = open('monty_python.csv', 'w')

In [3]: cfw = csv.writer(f, dialect='excel')

In [4]: cfw.writerow(['spam','spam','eggs','spam'])

In [5]: cfw.writerow(['spam','spam','spam and eggs', 'spam'])

In [6]: cfw.writerow(['toast'])

In [7]: f.close()

In [8]: cfr = csv.reader(open('monty_python.csv', 'rU'), dialect='excel')

In [9]: for row in cfr:
   ...:     print ', '.join(row)
spam, spam, eggs, spam
spam, spam, spam and eggs, spam

If you’d prefer a different separator than commas the delimiter optional argument can be used

In [1]: import csv

In [2]: f = open('tabbs.csv', 'rU')

In [3]: csvfile = csv.reader(f, delimiter='\t')

In [4]: for row in csvfile:
   ...:     print row
['1', '2', '3']
['2', '3', '4']
['4', '5', '6']

often you’ll want to skip the first row in a csv file, and a simple way to do that is

import csv

f = open('test_scores.csv', 'rU')
csvfile = csv.reader(f)

header = False

for row in csvfile:
   if not header:
      header = True
      print row

csv excercise

use the test_scores.csv file to calculate the average score (colum 4) for each sex (column 2)


The sys module contains many objects and functions for dealing with how python was compiled or called when executed. Most significantly is argv, which is a list containing all the parameters passed on the command line when python executed, including the name of the python program in position 0. Note that all the elements of sys.argv are strings - if you want a number, you will have to convert it using int() or float(). For example, if you want to assign the argument at position 1 as an integer variable, you can use n = int(sys.argv[1]).

import sys
if len(sys.argv) > 1:
  print sys.argv
  print 'no arguments passed'
[jacob@moku ~]$ python foo bar baz
['', 'foo', 'bar', 'baz']

sys.argv exercise

write a program that takes two arguments, your first name, and your age, and then prints out your name and the year you were born.

glob module

The glob module proves the glob function to perform file globbing similar to what the unix shell provides

In [1]: import glob

In [2]: for file in glob.glob('./*.txt'):
   ...:     print file

math module

The math module provides common algebra and trigonometric function along with several math constants. Note that the trigonometric functions work in radians rather than degrees. You can convert from radians to degrees with math.degrees and vice versa with math.radians.

In [1]: import math

In [2]: math.e
Out[2]: 2.718281828459045

In [3]: math.pi
Out[3]: 3.141592653589793

In [4]: math.log10(100)
Out[4]: 2.0

In [5]: math.log(math.e)
Out[5]: 1.0

In [7]: math.cos(math.pi)
Out[7]: -1.0

In [10]: math.exp(1)
Out[10]: 2.7182818284590455

In [11]: math.pow(5,2)
Out[11]: 25.0

In [12]: math.sin(math.pi)
Out[12]: 1.2246467991473532e-16

for those wondering about line 12, see floats

re (regular expressions)

The re module provides access to powerful regular expressions. Use to determine if a pattern exists in a string, or use re.findall to search for all instances of a pattern in a string.

In [1]: import re

In [2]: m ='games', 'all fun and games')

In [3]:
Out[3]: 'games'

In [4]: re.findall('[1234567890]+','12 drummers drumming, 11 pipers piping, 10 lords a-leaping')
Out[4]: ['12', '11', '10']

datetime and time

The datetime module provides time and datetime objects, allowing easy comparison of times and dates.

In [1]: import datetime

In [2]: a = datetime.time(8,30)

In [3]: b = datetime.time(9,45)

In [4]: b>a

The time module provides simple estimates for how long a command takes.

In [1]: import time

In [2]: a = time.time()

In [3]: time.sleep(10)

In [4]: b = time.time()

In [5]: print b-a

pickling and unpickling

The pickle module provides a way to save python objects to a file that you can unpickle later in a different program. This allows the saving and loading of complex objects such as classes, dictionaries etc.

In [1]: import pickle

In [2]: adict = {'a': [1,2,3], 'b' : [4,5,6]}

# pickle.dump takes as arguments the object to be pickled and a file handler
In [3]: pickle.dump(adict, open('adict.pic', 'w'))

# delete adict to see if loading from a pickled file works
In [4]: del adict

# adict has been deleted, so we get an error message
In [5]: adict
NameError                                 Traceback (most recent call last)
/Users/cliburn/hg/pcfb/<ipython-input-5-5f470a13239f> in <module>()
----> 1 adict

NameError: name 'adict' is not defined

# load adict from pickled file
In [6]: adict = pickle.load(open('adict.pic'))

In [7]: adict
Out[7]: {'a': [1, 2, 3], 'b': [4, 5, 6]}

pypi (formerly cheese shop)

The python community maintains a database of third party python packages. This database lives at pypi. Many of these packages can be installed using pip or easy_install. In the example below, I first use easy_install to install pip, then use pip to install xlrd (a module for reading XLS files). The -U flag means that the program should update the installation if the module is already installed. Finally, we can also use pip to uninstall modules (but not easy_install, which is why we prefer to use pip).

iMac:pcfb cliburn$ easy_install -U pip
Searching for pip
Installed /Users/cliburn/Library/Python/2.7/site-packages/pip-1.1-py2.7.egg
Processing dependencies for pip
Finished processing dependencies for pip

iMac:pcfb cliburn$ pip install -U xlrd
Downloading/unpacking xlrd from
Successfully installed xlrd
Cleaning up...

iMac:pcfb cliburn$ pip uninstall xlrd
Uninstalling xlrd:
Proceed (y/n)? y
  Successfully uninstalled xlrd


Write a program that takes a number on the command line and calculates the log, square, sin and cosine, and writes them out in a csv file.