Control Flow

Conditionals

if / elif / else

x = 10
 
if x > 0:
    print("positive")
elif x == 0:
    print("zero")
else:
    print("negative")

Note

Python uses indentation (4 spaces by default) to define code blocks instead of braces {}.

Ternary operator

A concise form of an if expression that returns a value.

x = 10
result = "positive" if x > 0 else "non-positive"

Loops

for

for i in range(5):
    print(i)    # 0, 1, 2, 3, 4

range()

Call	Description
range(stop)	0 to stop-1
range(start, stop)	start to stop-1
range(start, stop, step)	start to stop-1, incrementing by step

range(5)         # 0, 1, 2, 3, 4
range(2, 5)      # 2, 3, 4
range(0, 10, 2)  # 0, 2, 4, 6, 8
range(5, 0, -1)  # 5, 4, 3, 2, 1

Warning

range() does not include the stop value. range(5) goes from 0 to 4.

Iterating over collections

fruits = ["apple", "banana", "cherry"]
 
for fruit in fruits:
    print(fruit)
 
for index, fruit in enumerate(fruits):
    print(index, fruit)   # 0 apple, 1 banana, 2 cherry

Iterating backwards

my_list = [1, 2, 3]
 
for i in range(len(my_list) - 1, -1, -1):
    print(my_list[i])   # 3, 2, 1

while

count = 5
while count > 0:
    print(count)
    count -= 1

break, continue, else

for i in range(10):
    if i == 3:
        continue   # skip 3
    if i == 7:
        break      # exit loop at 7
    print(i)       # 0, 1, 2, 4, 5, 6

The for loop can have an else block that runs if the loop completes without break:

for i in range(5):
    if i == 10:
        break
else:
    print("loop completed without break")   # this runs

Nested loops

for i in range(3):
    for j in range(3):
        print(f"({i},{j})", end=" ")
    print()
# (0,0) (0,1) (0,2)
# (1,0) (1,1) (1,2)
# (2,0) (2,1) (2,2)

Next: Functions