Loops

Python has two loops available in syntax:

• for

• while

For

for <target> in <target_iterable>:
    <code>
else:
    <if_break_not_called_code>

Inside code it is possible to use continue and break:

• break statement terminates the loop without executing the else clause’s code

• continue statement skips the rest of code's statements and continues with the next item, or with the else clause if there is no next item

Notes:

• for fetches all items from iterable by firstly converting it to iterable by iter() then calling next() on it.

Example of usecase of "break/else":

courses = get_subscribed_courses(request.user)  # all courses of this user
for session in task.sessions.all():  # task.sessions - all sessions where this task assigned
    if session.course in courses:  # session belongs to assigned course - task is "valid"
        break
else:
    return redirect(reverse('list_hometasks'))  # redirect to the list of tasks

# Returning rendered page with task:
return render(request, template_name, context)

Here is a monster-kind example which aims to show all usecases of for loop. You can use it as a base for your experiments:

🪄 Code:

for x in range(1, 15):
    print(x, "\t: ", end="")
    if x in [8, 9, 10]:
        print("<---SKIPPING LINE--->")
        continue # immediately go on next iteration of x
    if x % 2:
        print("Even number", end="")
    else:
        print("Odd number", end="")
    if x == 12:
        print("\nBYE BYE!!! (break called!)")
        break # completely go out from for loop
    print(" ... EOL!") # will not triggered if 8 and 10
else:  # run this block only if no break statement called
    print("No break called -- number 12 was NOT FOUND")

📟 Output:

1 	: Even number ... EOL!
2 	: Odd number ... EOL!
3 	: Even number ... EOL!
4 	: Odd number ... EOL!
5 	: Even number ... EOL!
6 	: Odd number ... EOL!
7 	: Even number ... EOL!
8 	: <---SKIPPING LINE--->
9 	: <---SKIPPING LINE--->
10 	: <---SKIPPING LINE--->
11 	: Even number ... EOL!
12 	: Odd number
BYE BYE!!! (break called!)

One more example for for-else loop - selection of DB:

default = "master"
slaves = ["slave1", "slave2", "slave3"]
random.shuffle(slaves)

for slave in slaves:
    if self.is_alive(slave):
        selected = slave
        break
else:
    selected = default

Range function

range() returns iterable object needed for arithmetic progression. It is most often used in for loops. The arguments must be plain integers. If the step argument is omitted, it defaults to 1

range(start, stop[, step])

range(stop)

🪄 Code:

print(list(range(10)))
print(list(range(1, 11)))
print(list(range(0, 30, 5)))
print(list(range(0, 10, 3)))

📟 Output:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[0, 5, 10, 15, 20, 25]
[0, 3, 6, 9]

🪄 Code:

print(list(range(0, -10, -1)))
print(list(range(0)))
print(list(range(1, 0)))

📟 Output:

[0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
[]
[]

Also - from 3.5:

🪄 Code:

[*range(4)]

📟 Output:

[0, 1, 2, 3]

🪄 Code:

[1, 2, *range(3, 6), 6]

📟 Output:

[1, 2, 3, 4, 5, 6]

🪄 Code:

{*range(4), 1, 2}

📟 Output:

{0, 1, 2, 3}

Pythonic cycles

Try to avoid the code like the following one:

for i in range(len(food)):
    print(food[i])

Instead, use clearer "Pythonic" way:

for piece in food:
    print(piece)

Enumerate

enumerate(iterable, start=0)

Returns an enumerate object which will yield tuples index, item from given iterable (a sequence, an iterator, or some other object which supports iteration)

🪄 Code:

for index, girl in enumerate(["Olya", "Sveta", "Anna", "Maria"], start=1):
    print("Girl number {} is {}".format(index, girl))

📟 Output:

Girl number 1 is Olya
Girl number 2 is Sveta
Girl number 3 is Anna
Girl number 4 is Maria

The following code:

seasons = ['Spring', 'Summer', 'Fall', 'Winter']
list(enumerate(seasons))

list(enumerate(seasons, start=1))

is equivalent to:

def enumerate(sequence, start=0):
    n = start
    for elem in sequence:
        yield n, elem
        n += 1

While

while <condition>:
    <if_condition_True_code>
else:
    <if_break_not_called_code>

Inside statements it is possible to use continue and break:

• break statement terminates the loop without executing the else clause’s code

• continue statement skips the rest of the code's statements and goes back to testing the condition

🪄 Code:

while True: 
    n = int(input("Please enter an int: "))
    if n < 0: 
        continue # back to the start 
    elif n == 0: 
        break 
    print("Square is ", n ** 2) 
print("Goodbye")

📟 Output:

Please enter an int: 5
Square is  25
Please enter an int: 15
Square is  225
Please enter an int: 0
Goodbye