Loops in Python

The while Loop

Its general structure is:

while <expr>:
    <statement(s)>

The while loop is a loop that runs based on a condition. Just like if, elif, or else, the only thing a while loop cares about is whether the given condition is true. If it is true, it keeps running; if false, it does not — you cannot go wrong against structures like this; the moment you do, it’s the moment it’s done with you.

For example, let’s try going against a while loop below and see if it forgives us:

while False:
    print("You're only human — you can go against me, it's fine, I can keep running")

Run it and see whether it gives the result it’s thinking of — of course it won’t. You don’t get forgiveness if you go against structures like this.

Now let’s do the right things for it and see what it says:

while True:
    print("Thank you for treating me well — I'll always be your friend")

When you run the code above, it will keep running and being your friend until you cut the friendship (i.e., close it).

In my operators and print function article I discussed how other bool values are obtained, how Python understands them, and how every object or value has a bool value. If you replace the True or False in the examples above with expressions you’ve learned in that content, the loop will check the bool value of what you wrote — if it’s True it will keep running; if False it will stop — the same logic applies. Connect the topics you’ve learned rather than treating them as isolated subjects.

While Example

password = "123"
while (password != str(input("please enter your password >> "))):
    pass

Let’s examine the example above:

• We set a password

  password = “123”

• We wrote the condition: the entered text does not equal our password

  (password != str(input(“please enter your password >> ”))) as the condition

• If the condition holds — i.e., the password is incorrect — it will keep running.

• We used the pass keyword to keep running without doing anything.

• The moment the condition is satisfied — i.e., the passwords match — since we said “run while the condition is not correct”, when it becomes correct it stopped running.

If I had written the following and left it, I would have gotten an error — because what does the while loop do? I haven’t coded that part, so it would error:

password = "123"
while (password != str(input("please enter your password >> "))):

Instead of pass, we could also do the following:

password = "123"
while (password != str(input("please enter your password >> "))):
    "We are doing password verification with this loop"

Note: Inside loops you can use structures like if, elif, else, and other loops. Connect everything that has been taught, and remember that this is a language — languages are flexible and you are free to do what you want.

For example:

stop = False
count = 0
while not stop:
    count += 1
    print("count", count, "stop", stop)
    if count == 10:
        while count:
            print("count", count, "stop", stop)
            count -= 1
        stop = True
        print("count", count, "stop", stop)

The for Loop

The for loop is essentially an advanced while loop. Its general structure is:

for <variable_name> in <list or str or tuple>:
    <statement(s)>

In a for loop, the <list, str, tuple, or dict> part must be an iterator object.

How Iterators Work

>>> x = iter([1, 2, 3]) # x is now an iterator
>>> x
<listiterator object at 0x1004ca850>
>>> x.next() # each time we call the next function on our iterator object, it returns the next item
1
>>> x.next()
2
>>> x.next()
3
>>> x.next() # once items are exhausted, it raises an error
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

Here iter is a built-in function that takes an iterable object and returns an iterator.

This is actually the working logic of the for loop. You can also use a while loop to achieve the same effect as a for loop.

Examples of the for Loop

String Iterator

for i in "1234":
    print(i)

In each iteration, the value of i will be 1, 2, 3, and 4, in order.

As you can understand from this example, for loops run as many times as the length of the given iterator — they do not need a condition.

List Iterator

my_list = ["1", "2", "3"]
for i in my_list:
    print(i)

Here the number of elements is 3, so the for loop will run 3 times. On each run it calls the next() method on the iterator it created from ["1", "2", "3"], so i will be 1, 2, and 3 in order.

Dict Iterator

my_dict = dict(a=1, b=2, c=3)
for key, value in my_dict.items():
    print(key, value)

Here the number of elements is again 3, so the for loop will run 3 times. This time I’m using two variables called key, value because my iterator is a dictionary and each call to next will return a tuple with a key and its corresponding value.

Note: The i used in each loop is the letter i from “index”. It’s commonly used this way in general practice, but of course you can name the variable whatever you like.

Related Functions and Keywords

range

range is a built-in function that takes numbers as parameters and returns an iterator.

>>> for i in range(5):
...     print(i)
...
0
1
2
3
4

Below are additional examples and their outputs when substituted into the loop above:

range(5, 10)
   5, 6, 7, 8, 9

range(0, 10, 3)
   0, 3, 6, 9

range(-10, -100, -30)
  -10, -40, -70

Or an example where our variable type is list:

>>> a = ['Mary', 'had', 'a', 'little', 'lamb']
>>> for i in range(len(a)):
...     print(i, a[i])
...
0 Mary
1 had
2 a
3 little
4 lamb

The len function here returns the length (number of elements) of the variable passed to it as a parameter.

In each iteration, i becomes 0, 1, etc., and we access the elements of our list by writing a[i].

Note: If you put an iterator inside a list type, it converts it to a list.

>>> list(range(5))
[0, 1, 2, 3, 4]

break

The break keyword stops the loop directly above it.

break in a while Loop

count = 0
while True:
    count += 1
    print(count)
    if count == 10:
        break

In the example above, when the value of count reaches 10, the if conditional will run, and as a result, break will also run — closing (exiting) the while loop above it.

break in a for Loop

for i in range(100):
    print(i)
    if i == 10:
        break

This gives the same kind of result as the example above.

What if we use two loops?

count = 0
while True:
    count += 1
    print("count", count)
    for i in range(count):
        print("i", i)
        if i == 10:
            break

The break in this loop will stop the for loop when the condition is met, but since it cannot stop the outermost while loop, the while loop will keep running forever.

continue

continue carries the same meaning as the English word — it means “keep going”. Let’s explain it with an example in Python:

for i in range(20):
    if i == 10:
        continue
    print("this text won't print when i is 10", i)

In the example above, if i equals 10, we said “continue” — meaning: don’t look at the line below; execute the next value from the iterator.

else

Imagine we’re using an if conditional inside our for loop, but we want something like this: if this condition is never satisfied during the entire loop, run this at the end of the loop. That’s where else comes to the rescue again.

for i in range(20):
    if i == 21: # if this condition is never satisfied during the loop
        continue
else:
    print("run this line")

pass

We’ve already looked at the pass keyword — it comes to your aid whenever you want to say “do nothing here, but don’t leave this line empty because it will cause an error.”

Examples

In loops:

>>> while True:
...     pass  # Busy-wait for keyboard interrupt (Ctrl+C)
...

When defining a class:

>>> class MyEmptyClass:
...     pass
...

When defining a function:

>>> def initlog(*args):
...     pass   # Remember to implement this!
...