CS2304: Creating Modules and Packages - Virginia...

CS2304: Python for Java Programmers

Monti 2014

CS2304: Creating Modules and Packages


Monti 2014

Modules and Packages

•  At some point, you’ll likely want to create larger programs that span multiple files or provide your own library.

•  There are a few way to do that in Python: modules and packages.

•  Modules are simply Python files and are very straight forward to use. •  Normally, these are in the same directory as your “main”

program. •  Built-in modules also exist as part of the Python install.

•  Packages are a bit more complex, we’ll just briefly look at how you can use them.


Monti 2014

Using Modules

•  Modules are simply .py files, that can contain functions, types (classes), etc.

•  Let’s say module.py contains:

•  How do we go about using this module’s functions or variables?

MODULEVAR = 10 def get_name(): return "Hello"


Monti 2014

Enter the Import Statement

•  The import statement will let us access other modules.

•  Both when we are creating our own modules or using Python’s built-in modules.

•  Let’s say we want to use the module from last slide in the interactive interpreter:

>>>import module # now we use module functions >>>module.get_name() ‘Hello’ >>> module.MODULEVAR 10


Monti 2014

The Import Statement

•  The import statement has several variations. •  We can import specific functions, or everything at

once. •  Here’s another example, importing just a single

function:

>>>from module import get_name # I don’t need to qualify get_name with module >>>get_name() ‘Hello’ # However, this no longer works: >>> module.MODULEVAR 10


Monti 2014

The Import Statement

•  The import statement has several variations. •  Here’s another example, this one will import

everything* in the module:

•  * There are some exceptions we’ll talk about later.

>>>from module import * # I don’t need to qualify get_name with module >>>get_name() ‘Hello’ # This works again, without qualification : >>> MODULEVAR 10


Monti 2014

Another Variation Using “as”

•  You can also rename packages using import and the “as” statement.

•  Might do this when a name is too long:

>>> import module as m # Now I can refer module as m. >>>m.get_name() ‘Hello’ # This also works: >>> m.MODULEVAR 10 # Everything together, here I’ve renamed a function. >>> from module import get_name as name >>> name()


Monti 2014

A Final Word On Import

•  Which version will we be using?

•  The concern here is “flooding” the name space with duplicates.

•  Two or more names can collide. •  As an example, say there is a get_name() in moduleA and in moduleB? Which one is chosen?

>>> import module >>> module.get_name()

>>> from moduleA import * >>> from moduleB import *


Monti 2014

Creating / Using Packages

•  A package is simply a directory filled with Python modules and an additional __init__.py file.

•  The __init__.py file can actually be empty, or it can contain Python code.

•  __init__.py often contains a list called __all__, this is the list of variables and functions imported when using a “*”.

•  Let’s say you have a package Graphics, with a module called Jpeg.py, to import that module:

>>> import Graphics.Jpeg >>> Graphics.Jpeg.somefunction()


Monti 2014

Creating / Using Packages

•  Like with just modules you should avoid the *, but it can be used with packages as well:

•  You can also rename packages/modules as well:

•  These have the same name collision concerns as

modules.

>>> from Graphics import * >>> Jpeg.somefunction()

>>> import Graphics.Jpeg as Jpeg >>> Jpeg.somefunction()


Monti 2014

Standard Library Modules/Packages

•  The previous examples were written as if you were importing your own code.

•  The process is identical for any packages or modules that come with Python.

•  Take a look at Chapter 5 in the book for a tour of some of the stuff in the standard Python install.

•  There are built-in modules for: •  Handling command line options (optparse, argparse). •  Archiving/compressing files, so tar (tarfile), gzip, bz2, zipfile. •  Interacting with the file system, (os, os.path). •  Regular expressions (re), we’ll talk more about these later. •  There’s a large math library, we saw math.pi earlier.


Monti 2014

Back To This If Statement •  Remember this:

•  __name__ is defined for each module in your

program. •  Normally this just the name we imported:

def main(): print("Hello World”) if __name__ == "__main__": main()

>>> import module module.__name__ ‘module’


Monti 2014

Back To This If Statement

•  The one exception occurs if you don’t import a module, but rather “run” the module.

•  Instead of importing, running the module is what we are doing here:

•  I’m treating this module as the start point of my

program, so __name__ is set to “__main__” •  So what this is really saying is: when this module

is “run”, do what ever inside of the if statement:

if __name__ == "__main__": print(“I’m running”); main()

python3 myprogram.py


Monti 2014

Back To This If Statement

•  When I import the module, the if statement will never run because __name__ is “myprogram”:

•  So you can put an if statement like this at the

bottom of every module, and it will only run when you do something like this:

•  For example if you wanted to test code, you could

put something inside of the if statement:

if __name__ == "__main__": testFunction1(); testfunction2()

import myprogram

python3 module1


Monti 2014

Primitive Unit Testing With Doctest

if __name__ == "__main__": import doctest doctest.testmod()

def char_at(row, column): """Returns the character at the given position This is really just for debugging. >>> char_at(0, 0) '%' >>> char_at(4, 11) ' ' >>> char_at(32, 24) Traceback (most recent call last): ... RowRangeError """ # function code

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