Definition and use

Functions ensure the readability and reusability of your code:

• Factorize (regroup) statements that perform a specific task
• Be called on demand anywhere in your code
• Be tested independently to then be safely reused and shared

Algorithmic statements involving functions

• Defining functions (a function takes parameters and returns a value)
• Calling functions (associate values to parameters and get function’s output)

Defining functions in Python

• The name of a function, the number and types of its parameters, and the type of its returned value(s) is generally called the signature, or the prototype of a function.

def func_name (param_1: int, param_2: int) -> float:

    """
        This is an example of how to define a function in Python.
        Here, we use type hinting to indicate to the user how they should use it. 
        In:
            * param_1: The first argument to take as an input.
            * param_2: The second argument to take as an input.
        Out:
            * A result computed by the function.
    """

    # Do something
    ...

    # Return something
    return result

Calling a function in Python

• Once defined, a function can be called from anywhere just using its name and passing a value for each expected parameter
• Values given in a function call are called actual parameters or arguments.

def func_name (param_1: int, param_2: int) -> float:

    # Do something
    ...

    # Return something
    return result

# Call the function
res  = func_name(4, 2)

Importing modules

• Functions may be thematically regrouped into separated files (named modules), e.g., mymodule.py
• To use the functions located in mymodule.py, it has to be imported

# Import the functions from the file
# Alternate method: from mymodule import the_function_i_need
import mymodule

# Use the function
# With alternate method: the_function_i_need()
mymodule.the_function_i_need()

• When a file is imported it is also executed (to load functions in memory)
• To avoid some parts to be run when imported, use if __name__ == "__main__":

To keep code organized, it’s good practice to split it into multiple files:

• main.py – The main script
• functions.py – Contains reusable functions

The structure of a file such as functions.py should follow the following convention:

"""
    General description of the document contents.
"""

# Needed imports
# ...

# Various functions, well documented.
# ...

# Things to do when running this code directly
if __name__ == "__main__":
    # ...

Don’t reinvent the wheel!

• Python comes with a plethora of already defined functions regrouped into thematical modules
• Always favor the use of existing (official) modules (Python Standard Library + addional modules)

The python modules installer

• The Python Package Index (PyPI) is a repository of software for the Python programming language
• PyPI helps you find and install software developed and shared by the Python community, using the pip installer
• e.g., pip install matplotlib

Functions are reusable, named, blocks of code, that take parameters as input and produce a result

• Functions help making codes more readable and reusable

• They need to be defined before being used

• Coherent collections of functions should be grouped in modules

• Many modules already exist, and can be installed with pip

• Local variables exist in the function only

• Local variables should be favored over global ones