Python Modules

Python Modules
PYTHON MODULES
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A module allows you to logically organize your Python code. Grouping related code into a module
makes the code easier to understand and use. A module is a Python object with arbitrarily named
attributes that you can bind and reference.
Simply, a module is a file consisting of Python code. A module can define functions, classes and
variables. A module can also include runnable code.
Example
The Python code for a module named aname normally resides in a file named aname.py. Here's
an example of a simple module, support.py
def print_func( par ):
print "Hello : ", par
return
The import Statement
You can use any Python source file as a module by executing an import statement in some other
Python source file. The import has the following syntax:
import module1[, module2[,... moduleN]
When the interpreter encounters an import statement, it imports the module if the module is
present in the search path. A search path is a list of directories that the interpreter searches before
importing a module. For example, to import the module hello.py, you need to put the following
command at the top of the script −
#!/usr/bin/python
# Import module support
import support
# Now you can call defined function that module as follows
support.print_func("Zara")
When the above code is executed, it produces the following result −
Hello : Zara
A module is loaded only once, regardless of the number of times it is imported. This prevents the
module execution from happening over and over again if multiple imports occur.
The from...import Statement
Python's from statement lets you import specific attributes from a module into the current
namespace. The from...import has the following syntax −
from modname import name1[, name2[, ... nameN]]
For example, to import the function fibonacci from the module fib, use the following statement −
from fib import fibonacci
This statement does not import the entire module fib into the current namespace; it just introduces
the item fibonacci from the module fib into the global symbol table of the importing module.
The from...import * Statement:
It is also possible to import all names from a module into the current namespace by using the
following import statement −
from modname import *
This provides an easy way to import all the items from a module into the current namespace;
however, this statement should be used sparingly.
Locating Modules
When you import a module, the Python interpreter searches for the module in the following
sequences −
The current directory.
If the module isn't found, Python then searches each directory in the shell variable
PYTHONPATH.
If all else fails, Python checks the default path. On UNIX, this default path is normally
/usr/local/lib/python/.
The module search path is stored in the system module sys as the sys.path variable. The sys.path
variable contains the current directory, PYTHONPATH, and the installation-dependent default.
The PYTHONPATH Variable:
The PYTHONPATH is an environment variable, consisting of a list of directories. The syntax of
PYTHONPATH is the same as that of the shell variable PATH.
Here is a typical PYTHONPATH from a Windows system:
set PYTHONPATH=c:\python20\lib;
And here is a typical PYTHONPATH from a UNIX system:
set PYTHONPATH=/usr/local/lib/python
Namespaces and Scoping
Variables are names identifiers that map to objects. A namespace is a dictionary of variable names
keys and their corresponding objects values.
A Python statement can access variables in a local namespace and in the global namespace. If a
local and a global variable have the same name, the local variable shadows the global variable.
Each function has its own local namespace. Class methods follow the same scoping rule as
ordinary functions.
Python makes educated guesses on whether variables are local or global. It assumes that any
variable assigned a value in a function is local.
Therefore, in order to assign a value to a global variable within a function, you must first use the
global statement.
The statement global VarName tells Python that VarName is a global variable. Python stops
searching the local namespace for the variable.
For example, we define a variable Money in the global namespace. Within the function Money, we
assign Money a value, therefore Python assumes Money as a local variable. However, we accessed
the value of the local variable Money before setting it, so an UnboundLocalError is the result.
Uncommenting the global statement fixes the problem.
#!/usr/bin/python
Money = 2000
def AddMoney():
# Uncomment the following line to fix the code:
# global Money
Money = Money + 1
print Money
AddMoney()
print Money
The dir Function
The dir built-in function returns a sorted list of strings containing the names defined by a module.
The list contains the names of all the modules, variables and functions that are defined in a
module. Following is a simple example −
#!/usr/bin/python
# Import built-in module math
import math
content = dir(math)
print content
When the above code is executed, it produces the following result −
['__doc__', '__file__', '__name__', 'acos', 'asin', 'atan',
'atan2', 'ceil', 'cos', 'cosh', 'degrees', 'e', 'exp',
'fabs', 'floor', 'fmod', 'frexp', 'hypot', 'ldexp', 'log',
'log10', 'modf', 'pi', 'pow', 'radians', 'sin', 'sinh',
'sqrt', 'tan', 'tanh']
Here, the special string variable __name__ is the module's name, and __file__ is the filename from
which the module was loaded.
The globals and locals Functions −
The globals and locals functions can be used to return the names in the global and local
namespaces depending on the location from where they are called.
If locals is called from within a function, it will return all the names that can be accessed locally
from that function.
If globals is called from within a function, it will return all the names that can be accessed globally
from that function.
The return type of both these functions is dictionary. Therefore, names can be extracted using the
keys function.
The reload Function
When the module is imported into a script, the code in the top-level portion of a module is
executed only once.
Therefore, if you want to reexecute the top-level code in a module, you can use the reload
function. The reload function imports a previously imported module again. The syntax of the
reload function is this −
reload(module_name)
Here, module_name is the name of the module you want to reload and not the string containing
the module name. For example, to reload hello module, do the following −
reload(hello)
Packages in Python
A package is a hierarchical file directory structure that defines a single Python application
environment that consists of modules and subpackages and sub-subpackages, and so on.
Consider a file Pots.py available in Phone directory. This file has following line of source code −
#!/usr/bin/python
def Pots():
print "I'm Pots Phone"
Similar way, we have another two files having different functions with the same name as above −
Phone/Isdn.py file having function Isdn
Phone/G3.py file having function G3
Now, create one more file __init__.py in Phone directory −
Phone/__init__.py
To make all of your functions available when you've imported Phone, you need to put explicit
import statements in __init__.py as follows −
from Pots import Pots
from Isdn import Isdn
from G3 import G3
After you add these lines to __init__.py, you have all of these classes available when you import the
Phone package.
#!/usr/bin/python
# Now import your Phone Package.
import Phone
Phone.Pots()
Phone.Isdn()
Phone.G3()
When the above code is executed, it produces the following result −
I'm Pots Phone
I'm 3G Phone
I'm ISDN Phone
In the above example, we have taken example of a single functions in each file, but you can keep
multiple functions in your files. You can also define different Python classes in those files and then
you can create your packages out of those classes.
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