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Python Statements

In short, a statement is any valid Python instruction. Some statements (e.g. if statements, for statements, etc.) may contain other statements.

# The following are all statements:
5 

3 + 7 * 8

"Hello"

x = "hello"

print(x)

# Note: the following two lines are all one (1) if statement.
if True == False: # True == False is a statement on its own!
    print("The world is ending!") # this line is also a statement on its own, and so is the string "The world is ending!"!

Expressions​

Many statements can be evaluated to a value. For example:

1 + 2 * 3 ** 4

Will evaluate to the number that results from these mathematical operations. In this case, 163.

Statements that evaluate to a certain value are called expressions. Many statements in Python require expressions to be in certain places. For example:

Assignments​

One very common statement is an assignment. This is what lets you give names to certain expressions. Note that an assignment is not an expression; it does not have a value. An assignment follows this general syntax:

my_var = my_expr

Where my_var is the name you want to give to an expression, and my_expr is that very expression. my_expr must be an expression, or else an error will occur. Once this assignment statement is run, my_var becomes a name that refers to an object in memory. This object will have the value and type of my_expr. Another effect of this assignment is that my_var, as a name, becomes an expression. In fact, it becomes an expression that evaluates to the same value as my_expr. You can now use my_var in any place that requires an expression, like another assignment statement.

Control Flow​

In very simple Python scripts, every line gets executed by the interpreter. For example:

# Age Calculator

current_year = 2026
birth_year_string = input("Enter your birth year: ")
birth_year = int(birth_year_string)

age = current_year - birth_year
print(f'Your age is {age}!')

This script will always run line 1, then 2, then 3, etc. until it either gets to the end of the file or an error occurs. This is too rigid for most use cases. For example, what if the user does not give a valid number as a birth year? Surely we would want to run different lines of code depending on the circumstances.

"Control Flow" refers to the ability to control the "flow" of code in just this fashion. Sometimes we don't want every line of code to be run, and sometimes we want different lines of code to be run, and sometimes we even want the same lines of code to be run more than once. To accomplish this, we can use if, while, and for statements.

if Statements​

if statements allow you to only run some lines of code when a given expression is true. It looks like this:

if condition:
    suite

Here, condition can be any valid python expression. The type of the expression should usually be a bool, and this is what we will focus on for now. suite represents one or more statements that will only run when condition evaluates to True. These statements are given line by line as normal, and must be indented.

Indentation is how Python knows whether knows whether a statement following an if statement is a part of the suite, or if it is simply a statement after the if statement. This is also true of while and for statements below.

if condition:
    print("I'm in the if statement, I only run if condition is True.")
print("I'm outside of the if statement, I always run.")

conditions​

It is important to restate that the condition of an if statement can be any expression, but for most use cases we limit it to any expression that evaluates to a bool. This is where Boolean Operators can really shine, because they naturally result in bools!

Here are some possible

info

"Truthy" and "Falsy" values

If an if statement can accept any expression as a condition, how does it handle values that are not True or False? The answer is that every expression, no matter the type, has a "truthiness value", which is either "Truthy" or "Falsy". When an if statement receives a non-bool expression, it will use the truthiness value of the expression to choose whether or not to run its suite. In general, all values are "Truthy" except for the following, which are "Falsy":

  • None
  • False
  • Any number-typed value with a value of 0 (including 0.0)
  • Any empty sequence-type value ("", (), [])

else and elif​

while Statements​

AKA: while loops

for Statements​

AKA: for loops

Imports​