Loops allow to loop through the program code and repeat blocks of code until a given condition is fulfilled or not fulfilled. The code which is repeated is called the loop’s body.
This article is an excerpt of my book Python 3 in Anger. For a more detailed version take a look at the book.
Python has two types of loops: the
for loop and the
Loops have variables which change their values in every execution of the loop’s body and these variables have to be used as a condition either in the loop’s head or in the body to
break the flow.
If you do not have a terminating condition in your loop you might end up with an infinite or endless loop which never terminates and you have to kill your application manually. Most of these loops are achieved with
while constructs. Let me show you examples:
while True: print('I love Python!') i = 0 while i < 1: print('I love Python!')
In the fists example you have a condition in the head of the loop which evaluates always to
True thus the program will print
I love Python to the console forever. This loop would never terminate.
The second loop is a bit more tricky because I use a variable in the head of the loop. This loop would eventually terminate if the value of
i would be greater than
1 at one time. However because the loop’s body does only printing to the console this will never happen.
Let’s look at the loop types in detail to see how loops work and how we can avoid endless loops.
And one more nice thing about loops: they can handle an
else block which gets executed if the condition in the loop’s head is not fulfilled anymore. This is a new programming construct which seems strange to programmers of a classic programming language where the loops do not have an
Modifying the control flow in the loop’s body
Before we can go into the details of the loops we need to learn about two constructs which modify the loops’ control flow. These are
continue. Let’s see how they work in general and then with examples in the sections about the loops themselves.
break statement, as its name suggests, breaks out of the loop. Most of the time it is within a conditional block to stop the loop if it should not be continued anymore.
This statement ends the loop right away, so no more statements are executed — not in the loop’s body after the break statement and the
else part of the loop isn’t executed too.
while / for some_condition: execute_this do_this break this_does_not_executed this_neither else: this_is_leaved_behind this_too
In the example above I did not include any
if conditional to
break out of the loop to demonstrate the working of the statement.
Parallel to the
break statement you can tell the loop to
continue the execution at the head again. This means any other statement behind the
continue statement does not get executed, however the loop continues until the condition in the loop’s head evaluates to
False. In this case the
else block of the loop is executed when the lopp finished.
while / for some_condition: execute_this do_this continue this_does_not_executed this_neither else: this_is_executed_at_the_end this_too
Now we are ready to take a look at the loops themselves with examples. Are you ready? Why not?
for loop is designed to iterate over collections or to execute the loop’s body a finite amount of times. The general syntax of the loop looks like this:
for variable in sequence: statement_1 statement_2 ... statement_n else: else_statement_1 else_statement_2 ... else_statement_m
else block is optional as only one statement is required in the body of the
Let’s have a real example:
>>> for m in menu: ... print(m) ... else: ... print("What do you want?") ... eggs sausage bacon spam What do you want?
In the example above the
for loop iterates over the elements of the list and prints out every element to the console. After the loop is finished the
else block is executed once.
In the example above it seems really bothersome to define the list of numbers every time again-and-again. This is Python so there has to be some construct which leverages this from the developer. And there it is: the
The function requires one parameter: the stop value; and two optional parameters: start and step.
If only stop is provided then the
range function generates a range from the number
0 until the stop number (the stop number is not included) with the step of
>>> for i in range(10): ... print(i) ... 0 1 2 3 4 5 6 7 8 9
As you can see in the example above the numbers are printed from
9 with a step of 1.
If the start parameter is provided then the
range function creates a range between the start and the stop number (again, the stop number is exlusive).
If the start number is greater or equal than the stop number then the loop is not executed.
>>> for i in range(15,20): ... print(i) ... 15 16 17 18 19 >>> for i in range(25,10): ... print(i) ...
The step defines how many elements to leave out — or to step over the numbers between start and stop even if the start number is greater than the stop number. In this case you have to provide the step of
>>> for i in range(1,10,2): ... print(i) ... 1 3 5 7 9 >>> for i in range(10,1,-1): ... print(i) ... 10 9 8 7 6 5 4 3 2
As you can see, the
range function is used to execute the loop a finite amount of times.
for loop with
Now we know the basics of a
for loop let’s add the already known control-flow modifications:
As I have told you previously if you use break the whole loop is terminated. Do you remember the sample
for loop from the beginning of this section? Now let’s add a conditional to it to break when the element we iterate through is
>>> menu = ['eggs', 'sausage', 'bacon', 'spam'] >>> for m in menu: ... if m == 'spam': ... break ... print(m) ... else: ... print("What do you want?") ... eggs sausage bacon
>>> menu = ['eggs', 'sausage', 'bacon', 'spam'] >>> for m in menu: ... print(m) ... break ... else: ... print("What do you want?") ... eggs
Continue jumps back to the head of the loop in the case of a
for loop it jumps to the next element of the collection. If the collection is empty then the loop finishes and the
else loop is executed. Let’s see again two examples: one with a conditional continue and one with a continue of its own.
>>> menu = ['eggs', 'sausage', 'bacon', 'spam'] >>> for m in menu: ... if m == 'spam': ... continue ... print(m) ... else: ... print("What do you want?") ... eggs sausage bacon What do you want?
>>> menu = ['eggs', 'sausage', 'bacon', 'spam'] >>> for m in menu: ... continue ... print(m) ... else: ... print("What do you want?") ... What do you want?
As you can see, the main difference between
break is in the flow of the loop: the first one jumps back and iterates over the remaining elements, the second one terminates the whole loop. Naturally you can combine both together to have the right control flow of your own.
for loop with side-effects
Naturally you can have side-effects when using a
for loop with a list. That’s because the collection you iterate over is not immutable so you can change its value during the loop — which can result in unexpected behavior. Let’s see an example:
>>> l = ['eggs'] >>> for e in l: ... if e == 'eggs': ... l += ['sausage'] ... if e == 'sausage': ... l += 'spam' ... print(e) ... eggs sausage s p a m >>> l ['eggs', 'sausage', 's', 'p', 'a', 'm']
As you can see we modified the list during the execution of the for loop so when the loop finished and returned to the head for evaluation it found new elements in the list so the loop continued to execute.
To avoid this we can use a copy of the list to iterate over in the loop:
>>> l = ['eggs'] >>> for e in l[:]: ... if e == 'eggs': ... l += ['sausage'] ... if e == 'sausage': ... l += 'spam' ... print(e) ... eggs
while loop is designed to execute the loop’s body an indefinite amount of time until a condition is reached. With the
for loop you can execute the loop’s body only a finite amount of timmes (depending on the list or the
range you provide to it).
And because the
while loop need a conditional statement you can easily create an infinite loop (as seen in the introduction to loops).
Let’s see how a while loop is built up:
while condition_evaluates_to_True: statement_1 statement_2 ... statement_n else: else_statement_1 else_statement_2 ... else_statement_m
As you can see the structure is almost the same as by a
for loop however here you need a boolean condition wihch evaluates to
else block is the same and executes when the loop terminates normally. If a
break encounters in the loop’s body then the
else block is not executed.
>>> i = 0 >>> while i < 10: ... i += 1 ... else: ... print("Finished loop, i has the value of ", i) ... Finished loop, i has the value of 10
The main usage of a
while loop is in games or applications with user interaction for example where you need to get a specific type of input (for example a number) or want to execute logic until the game ended. Let’s see a simple example where we ask the user for numeric input.
while True: try: a = int(input('Enter a number: ')) except ValueError: print("This was not a number!") continue break print("You entered: ", a)
As you can see in this example, if the user does not enter a number then the application prints out “This was not a number!” and because of the
continue statement it executes the loop again. If the input can be converted tu a number then the
break statement terminates the endless
If you run the application you might get something like this:
Enter a number: enter This was not a number! Enter a number: a This was not a number! Enter a number: number This was not a number! Enter a number: 23j This was not a number! Enter a number: 42 You entered: 42
while loop with
>>> i = 0 >>> while i < 10: ... continue ... i += 1 ... else: ... print("Finished loop, i has the value of ", i) ...
This code won’t stop because
i is never incremented so after calling
continue the expression
i < 10 is evaluated again and again to
False so the loop executes infinitely. If you started the example above, you can stop it with hitting
CTRL-C on the keyboard.
>>> i = 0 >>> while i >> i 0
As you can see, when using
else block is not executed. And in the example above the value of
i did not change at all.
Naturally these are just basic examples with
continue, most of the time you use them in a conditional expression.