4.2 Defining Functions

  • square function that calculates the square of its argument.
In [1]:
def square(number):
    """Calculate the square of number."""
    return number ** 2
In [2]:
square(7)
Out[2]:
49
In [3]:
square(2.5)
Out[3]:
6.25
  • Calling square with a non-numeric argument like 'hello' causes a TypeError because the exponentiation operator (**) works only with numeric values

Defining a Custom Function

  • Definition begins with the (def keyword, followed by the function name, a set of parentheses and a colon (:).
  • By convention function names should begin with a lowercase letter and in multiword names underscores should separate each word.
  • Required parentheses contain the function’s parameter list.
  • Empty parentheses mean no parameters.
  • The indented lines after the colon (:) are the function’s block
    • Consists of an optional docstring followed by the statements that perform the function’s task.

Specifying a Custom Function’s Docstring

  • Style Guide for Python Code: First line in a function’s block should be a docstring that briefly explains the function’s purpose.

Returning a Result to a Function’s Caller

  • Function calls also can be embedded in expressions:
In [4]:
print('The square of 7 is', square(7))

The square of 7 is 49
  • Three Ways to Return a Result to a Function’s Caller
    • return followed by an expression.
    • return without an expression implicitly returns None—represents the absence of a value and evaluates to False in conditions.
    • No return statement implicitly returns None.

What Happens When You Call a Function

  • Parameters exist only during the function call.
  • Created on each call to the function to receive arguments.
  • Destroyed when the function returns its result to the caller.
  • A function’s parameters and variables defined in its block are all local variables.

Accessing a Function’s Docstring Via IPython’s Help Mechanism

  • Following a function's name with ? in IPython displays its docstring:
In [5]:
square?

Signature: square(number)
Docstring: Calculate the square of number.
File:      ~/Dropbox/books/2019/Python/PyCDS_JupyterSlides/ch04/<ipython-input-1-7d5dc51751d0>
Type:      function
  • If the function’s source code is accessible from IPython, ?? displays the function’s docstring and full source-code definition:
In [6]:
square??

Signature: square(number)
Source:   
def square(number):
    """Calculate the square of number."""
    return number ** 2
File:      ~/Dropbox/books/2019/Python/PyCDS_JupyterSlides/ch04/<ipython-input-1-7d5dc51751d0>
Type:      function

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