Tips for using python

Section author: Doug Wendel

If you are new to Python, this is the right place for you. This is not a comprehensive guide, rather this section is intended help you install an appropriate version of Python and get you started using the language with PyCogent.

Checking your version

Note

If you are running OSX 10.5 or later you only need to install the Apple Developer tools which come with OSX and you’ll have a suitable version.

At a minimum, you need to be using Python 2.5.x. If you have Python installed, open a terminal window and type python. The first few lines of text should tell you what version you’re running. For example:

Note

$ python Python 2.6.4 (r264:75706, Mar 11 2010, 12:48:01) [GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin Type “help”, “copyright”, “credits” or “license” for more information.

In this case, Python 2.6.4 is installed. If you don’t have Python installed or your version is older than 2.5.x, download the most recent 2.x release of Python here.

Warning

DO NOT install Python 3.x or above. This version of the language was significantly restructured and will currently not work with PyCogent.

Getting help

Python comes with a built-in help utility. To access it, open a terminal window and type python to enter Python’s interactive mode:

help()

Welcome to Python 2.6!  This is the online help utility.

If this is your first time using Python, you should definitely check out
the tutorial on the Internet at http://docs.python.org/tutorial/.

Enter the name of any module, keyword, or topic to get help on writing
Python programs and using Python modules.  To quit this help utility and
return to the interpreter, just type "quit".

To get a list of available modules, keywords, or topics, type "modules",
"keywords", or "topics".  Each module also comes with a one-line summary
of what it does; to list the modules whose summaries contain a given word
such as "spam", type "modules spam".

help>

Note that you are now in the interactive help mode as noted by the prompt help>. In this mode you can type in the names of various objects and get additional information on them. For example, if I want to know something about the map function:

help> map

Help on built-in function map in module __builtin__:

map(...)
map(function, sequence[, sequence, ...]) -> list

Return a list of the results of applying the function to the items of
the argument sequence(s).  If more than one sequence is given, the
function is called with an argument list consisting of the corresponding
item of each sequence, substituting None for missing values when not all
sequences have the same length.  If the function is None, return a list of
the items of the sequence (or a list of tuples if more than one sequence).
(END)

To exit the interactive help mode, simply enter a blank line at the help> prompt:

help>

You are now leaving help and returning to the Python interpreter. If you want to ask for help on a particular object directly from the interpreter, you can type “help(object)”. Executing “help(‘string’)” has the same effect as typing a particular string at the help> prompt.

As the parting message suggests, you can also invoke help on a specific object directly:

help(abs)

Help on built-in function abs in module __builtin__:

abs(...) abs(number) -> number

Return the absolute value of the argument. (END)

To quit help in this case just press q.

Using the dir() function

Another useful built-in function is dir(). As the name implies, it’s use is to list defined names in the current scope. To list the currently defined names:

dir() [‘__builtins__’, ‘__doc__’, ‘__name__’, ‘__package__’]

The list shows which names are currently defined. This list includes all imported modules and variable names. For example, if I define a new variable, it will also show up in this list:

my_variable = ‘Just testing’ dir() [‘__builtins__’, ‘__doc__’, ‘__name__’, ‘__package__’, ‘my_variable’]

Imported modules will also be reflected in this list:

import os
import sys
dir()
['__builtins__', '__doc__', '__name__', '__package__', 'my_variable', 'os', 'sys']

dir() can also be used to list the names defined within a module:

import sys
dir(sys)
['__displayhook__', '__doc__', '__excepthook__', '__name__', '__package__', '__stderr__', '__stdin__', '__stdout__',...

It also works on variable types. For example, let’s see what attributes the string class has as defined:

dir(str)
['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', '__format__', '__ge__',...

You can also use dir() on a defined variable. It will inspect the variable’s type and report the attributes for that type. In this case, we defined a variable my_variable of type str. Calling dir(my_variable) will product the same result as calling dir(str):

my_variable = 'Just testing'
dir(my_variable)
['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', '__format__', '__ge__',...

Hello PyCogent!

Now that we’ve gotten our feet wet, let’s write a simple function that returns a friendly message. This is a simple function which takes in one parameter, your_name, and outputs the user’s name prefixed with a standard message. Calling your new function is as simple as typing the name of the function and supplying the appropriate variables:

>>> def hello_pycogent(your_name):
...     message = 'PyCogent bids you welcome ' + your_name
...     print message
...
>>> hello_pycogent('John Smith')
PyCogent bids you welcome John Smith

Enter each line as you see it and note that white space is important! There are no brackets or keywords to signal blocks of code. Instead, indentation is used to designate related lines of code.

Further Python documentation

Now that you’ve got Python up and running and know a few commands, it might be useful to browse the official documentation. There is a comprehensive list of information and some excellent tutorials to work though. There are also many code examples to be found in the Python cookbook.