Python Notes (3) - Functions

The note covers funtion syntax, importing modules and build-in functions.

Python notes of open courses @Codecademy.

About Functions

If you need reuse a piece of code, just with a few different values. Instead of rewriting the whole code, it's much cleaner to define a function, which can then be used repeatedly.

Function Syntax

• Function Junction:

  • Functions are defined with three components:

    • Header: which includes the def keyword, the name of the function, and any parameters the function requires.

    • Comment: An optional comment that explains what the function does.

    • Body: which describes the procedures the function carries out. The body is indented, just like for conditional statements.

      # Example of a function
      def hello_world():
          """Prints 'Hello World!' to the console."""
          print "Hello World!"
      

• Call and Response:

  • function(parameters)
  • return sth

• Parameters and Arguments:

  • def square(n):
    • n is a parameter of square.
    • A parameter acts as a variable name for a passed in argument.
  • parameter = formal parameter (形参)， argument = actual parameter (实参)。
  • A function can require as many parameters as you'd like, but when you call the function, you should generally pass in a matching number of arguments.

• Functions Calling Functions

  • A function can call another function, in the way mentioned above.

Importing Modules

• A module is a file that contains definitions — including variables and functions — that you can use once it is imported.

• Generic Imports: import module

  • There is a Python module named math that includes a number of useful variables and functions, and sqrt() is one of those functions.
  • When it's simply done with the import module and module.function() to access math module in the way below, it's called a generic import.

• Function Imports: from module import function

  • Pulling in just a single function from a module is called a function import, and it's done with the from keyword.

• Universal Imports: from module import *

  • Use the power of from module import * to import everything from the math module.

  • However, they fill your program with a ton of variable and function names without the safety of those names still being associated with the module(s) they came from.

    # Examples of the above importing ways
    
    # Generic import
    import math
    print math.sqrt(25)
    
    # Function import
    from math import sqrt
    print sqrt(25)
    
    # Universal import
    from math import *
    print sqrt(25)
    

• Tips:

  • A generic import may be the best way.

  • You may see everything in a module by:

    import math            # Imports the math module
    everything = dir(math) # Sets everything to a list of things from math
    print everything       # Prints them all!
    

Built-in Functions

• We may directly use some of the functions that are built in to Python (no modules required!).
• max():
  • takes any number of arguments and returns the largest one.
• min():
  • returns the smallest of a given series of arguments.
• abs():
  • returns the absolute value of the number it takes as an argument—that is, that number's distance from 0 on an imagined number line.
  • It only takes a single number, unlike max() and min().
• type():
  • returns the type of the data it receives as an argument.

External Resources

• Functions in Python
• Functions Can Return Something