Statements

Statements are the building blocks of Python programs: control flow, function definitions, imports, and exception handling.

print

The print() function outputs values to the console.

>>> s = "Hello world"
>>> print(s)
Hello world

Note

In Python 3, print is a function and requires parentheses: print("text"). The Python 2 syntax print "text" no longer works.

for

The for statement iterates over the elements of a sequence.

x = [1, 2, 3, 4, 5, 6]
for numb in x:
    print(numb + 1)

if / elif / else

Conditional execution based on boolean expressions.

x = [1, 2, 3, 4, 5, 6]
for numb in x:
    if numb < 3:
        print("lower")
    elif numb == 3:
        print("equal")
    else:
        print("higher")

Warning

The else clause does not take a condition. If you need another condition, use elif.

while

Repeated execution as long as a condition is true.

count = 0
while count < 9:
    print(f"The count is: {count}")
    count += 1

def / return

Define a reusable function with def. Use return to send a value back to the caller.

def divide(x, y):
    if y != 0:
        return x / y
    return 0

Note

Use != instead of <> for “not equal” — the <> operator was removed in Python 3. Also note that def and if must be lowercase.

try / except / else

Handle exceptions gracefully with try/except.

def divide(x, y):
    try:
        result = x / y
    except ZeroDivisionError:
        print("Division by zero!")
    else:
        print(f"Result is {result}")

del

Delete a variable, list item, or slice.

>>> a = [-1, 1, 66.25, 333, 333, 1234.5]
>>> del a[0]
>>> a
[1, 66.25, 333, 333, 1234.5]
>>> del a[:]
>>> a
[]
>>> del a
>>> a
NameError: name 'a' is not defined

import

Import modules to extend Python’s functionality.

import os
import time
from pathlib import Path
import numpy as np

eval

eval() evaluates a string as a Python expression and returns the result.

>>> x = eval("5")
>>> x
5
>>> eval("2 + 3")
5

Warning

Avoid using eval() with untrusted input — it can execute arbitrary code.

exec

exec() executes a string containing Python statements.

>>> exec('x = 5')
>>> x
5

Warning

Like eval(), avoid exec() with untrusted input. In most cases there are better alternatives (dictionaries, lists, or dedicated data structures).