ifStatement and Conditionals
if in Python means: only run the rest of this code once, if the condition evaluates to
True. Don’t run the rest of the code at all if it’s not.
Anatomy of an
if statement: Start with the
if keyword, followed by a boolean value, an expression that evaluates to
True, or a value with “Truthiness”. Add a colon
:, a new line, and write the code that will run if the statement is
True under a level of indentation.
Remember, just like with functions, we know that code is associated with an
if statement by it’s level of indentation. All the lines indented under the
if statement will run if it evaluates to
>>> if 3 < 5: ... print("Hello, World!") ... Hello, World!
if statements only run if the expression in them evaluates to
True and just like with functions, you’ll need to enter an extra space in the REPL to run it.
If you only want your code to run if the expression is
False, use the
>>> b = False >>> if not b: ... print("Negation in action!") ... Negation in action!
ifStatements and Truthiness
if statements also work with items that have a “truthiness” to them.
>>> message = "Hi there." >>> a = 0 >>> if a: # 0 is False-y ... print(message) ... >>> b = -1 >>> if b: # -1 is Truth-y ... print(message) ... Hi there. >>> c =  >>> if c: # Empty list is False-y ... print(message) ... >>> d = [1, 2, 3] >>> if d: # List with items is Truth-y ... print(message) ... Hi there.
ifStatements and Functions
You can easily declare
if statements in your functions, you just need to mindful of the level of indentation. Notice how the code belonging to the
if statement is indented at two levels.
>>> def modify_name(name): ... if len(name) < 5: ... return name.upper() ... else: ... return name.lower() ... >>> name = "Nina" >>> modify_name(name) 'NINA'
Using the same technique, you can also nest your
>>> def num_info(num): ... if num > 0: ... print("Greater than zero") ... if num > 10: ... print("Also greater than 10.") ... >>> num_info(1) Greater than zero >>> num_info(15) Greater than zero Also greater than 10.
Remember, comparisons in Python evaluate to
False. With conditional statements, we check for that value implicitly. In Python, we do not want to compare to
Warning - pay attention, because the code below shows what you shouldn’t do.
# Warning: Don't do this! >>> if (3 < 5) == True: # Warning: Don't do this! ... print("Hello") ... Hello # Warning: Don't do this! >>> if (3 < 5) is True: # Warning: Don't do this! ... print("Hello") ... Hello
Do this instead:
>>> if 3 < 5: ... print("Hello") ... Hello
If we want to explicitly check if the value is explicitly set to
False, we can use the
>>> a = True # a is set to True >>> b = [1, 2, 3] # b is a list with items, is "truthy" >>> >>> if a and b: # this is True, a is True, b is "truthy" ... print("Hello") ... Hello >>> if a is True: # we can explicitly check if a is True ... print("Hello") ... Hello >>> if b is True: # b does not contain the actual value of True. ... print("Hello") ... >>>
else statement is what you want to run if and only if your
if statement wasn’t triggered.
else statement is part of an
if statement. If your
if statement ran, your
else statement will never run.
>>> a = True >>> if a: ... print("Hello") ... else: ... print("Goodbye") ... Hello
>>> a = False >>> if a: ... print("Hello") ... else: ... print("Goodbye") ... Goodbye
In the REPL it must be written on the line after your last line of indented code. In Python code in a file, there can’t be any other code between the
if and the
SyntaxError: invalid syntax if you try to write an
else statement on its own, or put extra code between the
if and the
else in a Python file.
>>> if a: ... print("Hello") ... Hello >>> else: File "<stdin>", line 1 else: ^ SyntaxError: invalid syntax
elifMeans Else, If.
elif means else if. It means, if this
if statement isn’t considered
True, try this instead.
You can have as many
elif statements in your code as you want. They get evaluated in the order that they’re declared until Python finds one that’s
True. That runs the code defined in that
elif, and skips the rest.
>>> a = 5 >>> if a > 10: ... print("Greater than 10") ... elif a < 10: ... print("Less than 10") ... elif a < 20: ... print("Less than 20") ... else: ... print("Dunno") ... Less than 10