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How to configure and use the Python interpreter

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Introduction to the Python interpreter

The interactive prompt

The interactive prompt allows interacting with the Python interpreter from a terminal. Once the interactive prompt has been launched, it is possible to enter valid Python code line by line. A line of the interactive prompt always starts with the characters >>> or ... depending on indentation.

$ python
Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56)
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> prompt = "guest"
>>> if prompt:
...     print prompt

To exit from the Python interactive prompt, we'll hit Ctrl+D under linux ubuntu. Since the interactive prompt plays an important role in the development of Python code, we would customize it for a more efficient use. This is explained in the following paragraph.

The start-up file of the Python interactive prompt

The interactive prompt can be configured with a start up file. When firing it up, the interactive prompt search the environment variable PYTHONSTARTUP and executes the code which is into the file pointed to by this variable. Some Linux distributions provide a default start up script, which is usually located in the home directory; this script is called .pythonstartup. Features, such as words completion by the TAB key, command history, are often required to improve the interactive prompt functionality. These features are based on the module readline, which is therefore required. It is also possible to create a simple start up script as it follows:

import readline
import rlcompleter
import atexit
import os
# tab completion
readline.parse_and_bind('tab: complete')
# history file
histfile = os.path.join(os.environ['HOME'], '.pythonhistory')
except IOError:
atexit.register(readline.write_history_file, histfile)
del os, histfile, readline, rlcompleter

In a Linux system, the easiest way is to create the start up script in the home directory and call it .pythonstartup. Then we can add a system environment variable PYTHONSTARTUP which would point to the .pythonstartup script. This can be done, for example, by using the .profile (or rather .bashrc), where we can initialize the variable PYTHONSTARTUP by inserting a line like the following one:

export PYTHONSTARTUP=~/.pythonstartup

When the interactive prompt is launched, the script .pythonstartup should be executed and the new features become available. For example, the words completion features by the TAB key is really useful to introspect all attributes of any Python object:

>>> import hashlib
>>> hashlib.<tab>
hashlib.__class__(                  hashlib.__loader__       hashlib.__subclasshook__(
hashlib.__delattr__(                hashlib.__name__         hashlib._hashlib
hashlib.__dict__                    hashlib.__new__(         hashlib.md5(
hashlib.__doc__                     hashlib.__package__      hashlib.new(
hashlib.__file__                    hashlib.__py_new(        hashlib.sha1(
hashlib.__format__(                 hashlib.__reduce__(      hashlib.sha224(
hashlib.__get_builtin_constructor(  hashlib.__reduce_ex__(   hashlib.sha256(
hashlib.__getattribute__(           hashlib.__repr__(        hashlib.sha384(
hashlib.__hash__(                   hashlib.__setattr__(     hashlib.sha512(
hashlib.__hash_new(                 hashlib.__sizeof__(

An advanced Python interactive interpreter

We can adapt the start up script .pythonstartup for more automation. In addition, the module code provides the base classes of the interpreter. However there is already an advanced Python interactive interpreter, called IPython, with features such as dynamic introspection of objects, system terminal access from the Python prompt, debugging tools, etc..

Launch the Python interactive interpreter from the system terminal

Launching the Python interactive interpreter from the system terminal (by simply editing python), gives access to a certain number of options:

$ python [-dEiOQsStuvVxX3?] [-c command | -m module-name | script | - ] [args]
$ python -V
Python 2.6.5

Any arguments following the python command is available from the attribute sys.argv, with for instance the first element available at sys.argv[0]. The most common usage of the Python interpreter is, of course, a simple invocation of the filename of a Python script. In this case, the Python interpreter reads and executes the script from that file:

$ python script.py [args]

When called with the -c command, it executes the instructions data given in place of command. The set of instructions referred to by command may contain multiple lines separated by line breaks, taking into account the necessary indentation spaces. If this option is used, the first element of sys.argv will be -c and the current directory will be added at the beginning of sys.path, allowing modules in that directory to be imported.

$ python -c """
> a=2; b=3
> if a: print a+b"""

When the Python interpreter is called with -m module-name, the given module is located on the Python modules import paths and is run as a script. The module name given does not contain the .py extension.

$ python -m module [args]

With the -i option we can, all at once, launch a python script, run it, start the Python interactive interpreter and access the attributes of the script. For example, let's consider a Python script file in the current directory, named interpreter.py, containing the following script:

# -*- coding: utf-8 -*-
# Copyright 2012 RasadaCrea. All rights reserved
a_string = 'string'
an_integer = 11
an_arg = 'an_arg_of_function'
def a_function(an_arg):
    return an_arg
class AClass:
an_instance = AClass()
print '%s - %d - %s - %s' % (a_string, an_integer, a_function(an_arg), an_instance)

Launching this script, from the terminal, with the -i option of the python command, we could get the following experience:

$ python -i interpreter.py
string - 11 - an_arg_of_function - <__main__.AClass instance at 0xb738ca0c>
>>> vars()
{'an_integer': 11, '__builtins__': <module '__builtin__' (built-in)>,
 'AClass': <class __main__.AClass at 0xb7382a7c>, '__package__': None,
 'an_instance': <__main__.AClass instance at 0xb738ca0c>,
 'an_arg': 'an_arg_of_function', '__name__': '__main__', '__doc__': None,
 'a_string': 'string', 'a_function': <function a_function at 0xb737da74>}
>>> an_integer
>>> a_function
<function a_function at 0xb737da74>
>>> a_function('another argument')
'another argument'
>>> an_instance
<__main__.AClass instance at 0xb738ca0c>

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