Python control flow tools

The if, else, and elif statements control conditional code execution. As one would expect, the format of the conditional statement is as follows:

    if expression:
      do something
    elif expression:
      do something if the expression meets
    elif expression:
      do something if the expression meets
    ...
    else:
      statement

Here is a simple example:

    >>> a = 10
    >>> if a > 1:
    ...   print("a is larger than 1")
    ... elif a < 1:
    ...   print("a is smaller than 1")
    ... else:
    ...   print("a is equal to 1")
    ...
    a is larger than 1
    >>>

The while loop will continue to execute until the condition is false, so be careful with this one if you don't want to continue to execute:

    while expression:
      do something

    >>> a = 10
    >>> b = 1
    >>> while b < a:
    ...   print(b)
    ...   b += 1
    ...
    1
    2
    3
    4
    5
    6
    7
    8
    9

The for loop works with any object that supports iteration; this means all the built-in sequence types such as lists, tuples, and strings can be used in a for loop. The letter i in the following for loop is an iterating variable, so you can typically pick something that make sense within the context of your code:

    for i in sequence:
      do something

    >>> a = [100, 200, 300, 400]
    >>> for number in a:
    ...   print(number)
    ...
    100
    200
    300
    400

You can also make your own object that supports the iterator protocol and be able to use the for loop for this object: