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The if Statement and Code Branching

Lesson 8

Python, like most programming languages, has an if statement that provides branching in your code. The syntax of Python’s if statement is as follows:

if [expression is True]:
    # execute this code when True 
else:
    # execute this code when False

The else branch is optional:

if [expression is True]:
    # only executes when True
# resume code execution

The expression can be anything that evaluates to True or False. A few examples:

  1. if num >= 10:
  2. if str == "Hello:
  3. if this != that:
  4. if SomeVar:

… and so on.

Take note of example 4 above—in Python, anything that does not equate to zero, Null, or an empty object is True. For example:

>>> s = 0
>>> if s:
...    print("True")
...    # Python returns nothing - statement is false
>>> s = 1
>>> if s:
...    print("True")
...
True
>>> s = ""
>>> if s:
...    print("True")
...    # Nothing again - statement is false
>>> s = "Hello"
>>> if s:
...    print("True")
...
True

Python includes a comprehensive range of boolean operators that you can use within your expressions:

  • <. Less than
  • <=. Less than or equal
  • >. Greater than
  • >=. Greater than or equal
  • ==. Equal
  • !=. Not equal
  • is. Is a particular object
  • is not. Isn’t a particular object

Boolean operations are also supported for negating and chaining expressions:

  • or. Either expression can be True
  • and. Both expressions must be True
  • not. Negate the proceeding expression

Python also supports multiple branching using the elif (short for “else if “) statement:

if [exp1 is True]:
    # execute if exp1 is True 
elif [exp2 is True]:
    # execute if exp2 is True 
elif [exp3 is True]:
    # execute if exp3 is True
# and so on ...

Lesson Resources

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