First Python Program

2.1 Diving in

Example 2.1. odbchelper.py

def buildConnectionString(params):
    """Build a connection string from a dictionary of parameters.

    Returns string.
    """
    return ";".join(["%s=%s" % (k,v) for k,v in params.items()])

if __name__ = "__main__":
    myParams = {"server":"mpilgrim",\
                "database":"master",\
                "uid":"sa",\
                "pwd":"secret"
    }
    print buildConnectionString(myParams)

Output:

server=mpilgrim;uid=sa;database=master;pwd=secret

2.2 Documenting Functions

  • You can document a Python function by giving it a doc string.

  • Triple quotes “”“ used when defining a doc string.

    • Triple quotes are useful to define a string with both single and double quotes.
    • Doc string, if it exists, must be the first thing defined in a function (that is, the first thing after the colon).

2.3 Everything is an object

Example 2.3. Accessing the buildConnectionString Function’s doc string

In [1]: import odbhelper

In [2]: params = {"server":"mpilgrim","database":"master","uid":"sa","pwd":"secret"}

In [3]: print odbhelper.buildConnectionString(params)
Out[3]: server=mpilgrrim;uid=sa;database=master;pwd=secret

In [4]: print odbhelper.buildConnectionString.__doc__
Out[4]: Build a connection string from a dictionary

        Returns string.
  • when you want to use functions defined in imported modules, you need to include the module name.

2.4 The import Search Path

Example 2.4. Import Search Path

In [1]: import sys

In [2]: sys.path
Out[2]: ['',
 '/usr/local/lib/python2.7/dist-packages/pymc-2.2-py2.7-linux-i686.egg',
 '/usr/local/lib/python2.7/dist-packages/Jinja2-2.6-py2.7.egg',
 '/usr/lib/python2.7',
 '/usr/lib/python2.7/plat-linux2',
 '/usr/lib/python2.7/lib-tk',
 '/usr/lib/python2.7/lib-old',
 '/usr/lib/python2.7/lib-dynload',
 '/usr/local/lib/python2.7/dist-packages',
 '/usr/local/lib/python2.7/dist-packages/PIL',
 '/usr/lib/python2.7/dist-packages',
 '/usr/lib/python2.7/dist-packages/PIL',
 '/usr/lib/python2.7/dist-packages/gst-0.10',
 '/usr/lib/python2.7/dist-packages/gtk-2.0',
 '/usr/lib/pymodules/python2.7',
 '/usr/lib/python2.7/dist-packages/ubuntu-sso-client',
 '/usr/lib/python2.7/dist-packages/ubuntuone-client',
 '/usr/lib/python2.7/dist-packages/ubuntuone-control-panel',
 '/usr/lib/python2.7/dist-packages/ubuntuone-couch',
 '/usr/lib/python2.7/dist-packages/ubuntuone-storage-protocol',
 '/usr/lib/python2.7/dist-packages/wx-2.8-gtk2-unicode',
 '/usr/lib/python2.7/dist-packages/IPython/extensions']

In [3]: sys
Out[3]: <module 'sys' (built-in)>
In [4]: sys.path.append(os.path.abspath('sphinxext'))
  • sys.path is a list of directory names that constitute the current search path. Python will look through these directories (in this order) for a .py file matching the module name you’re trying to import.
  • You can add a new directory to Python’s search path at runtime by appending the directory name to sys.path, and then Python will look in that directory as well, whenever you try to import a module. (i.e., here I try to add the sphinx extension directory to the search path)
  • everything in Python is an object. Strings are objects. Lists are objects. Functions are objects. Even modules are objects.

2.5 Indenting Code

Example 2.5. Indenting the fib Function

def fib(n):
    # you can mix and match to print several thing on one line
    # by using a comma-separated list of values.
    # Each value is printed on the same line, separated by spaces (i.e., "n = 5")
    print 'n = ', n
    if n > 1:
         return n * fib(n-1)
    else:
         print 'end of the line'
         return 1

2.6 Testing Modules

Python modules are objects and have several useful attributes. You can use this to easily test your modules as you write them.

if __name__ = "__main__":

  • parentheses are not required around the if expression

  • all modules have a built-in attribute __name__, A module’s __name__ depends on how you’re using the module.

    • If you import the module, then __name__ is the module’s filename, whithout a directory path or file extension
    • You can also run the module directly as a standalone program, in which case __name__ will be a special default value, __main__
In [1]: import odbhelper
In [2]: odbhelper.__name__
Out[2]: 'odbhelper'
  • knowning this, you can design a test suite for your module within the module itself by putting it in this if statement. When you run the module directly, __name__ is __main__, so the test suite executes. When you import the module, __name__ is something else, so the test suite is ignored. This makes it easier to develop and debug new modules before integrating them into a larger program.