An alternative way of implementing loops is by using a for loop. for loops tends to be easier and safer to use than while loops, but cannot be applied to all iteration problems. while loops are more general. In other words, everything that a for loop can do, a while loop can do too, but not the other way around.

The syntax of a for loop is as follows:

for <variable> in <collection>:
    <statements>

A for loop gets presented with a collection of items, and it will process these items, in order, one by one. Every cycle through the loop will put one item in the variable given next to the for, and can then be used in the code block under the for. The variable does not need to exist before the for loop is encountered. If it does, it gets overwritten. It is a real variable, by the way, in the sense that it still exists after the loop has finished. It will contain the last value that it got assigned during the processing of the loop.

At this point you might wonder what a “collection” is. There are many different kinds of collections in Python, and in this section I will introduce a few. In later chapters collections will be discussed in more detail.

for loop with strings

The only collection introduced until now is the string. A string is a collection of characters, e.g., the string "banana" is a collection of the characters "b", "a", "n", "a", "n", and "a", in that specific order. The following code loops through each of these letters:

for letter in "banana":
    print( letter )
print( "Done" )

While this code is fairly trivial, let’s go through it step by step (I did not make a flow chart, as that is not easy for for loops).

When the for loop is encountered, Python takes the collection (i.e., the string "banana") and turns it into an “iterable.” What that is exactly I will get to in Chapter 24, but for now assume that it is a list of all the letters in the string, in the order that they appear in the string. Python then takes the first of those letters, and puts it in the variable letter. It then executes the code block below the for.

The code block contains only one statement, which is the printing of letter. So the program prints “b,” and then loops back to the for. Python then takes the next letter, which is an “a,” and it executes the code block with letter being "a". It then repeats this process for each of the remaining letters. Once all the letters have been used, the for loop ends, and Python executes the last line of the code, which is the printing of the word “Done.”

To be absolutely clear: In a for loop you do not have to write code that explicitly increases some kind of variable that then grabs the next letter, or something like that. The for statement handles that automatically: every time it is looped back to, it takes the next item from the collection.

for loop using a variable as collection

In the code above, the literal string "banana" was used as the collection, but it could also be a variable that contains a string. For instance, the following code runs similar to the previous code:

fruit = "banana"
for letter in fruit:
    print( letter )
print( "Done" )

You might wonder if this isn’t dangerous. What happens if the programmer changes the contents of the variable fruit in the loop’s code block? You can try this out using the following code:

fruit = "banana"
for letter in fruit:
    print( letter )
    if letter == "n":
        fruit = "orange"
print( "Done" )

As you can see when you run this code, changing the contents of the variable fruit in the loop has no effect on the loop’s processing. The sequence of characters that the loop processes is only constituted once, when the for loop is first entered. Changing the value of fruit into "orange" while the loop is still processing the value "banana", does not stop it from continuing to process "banana". This is a great feature of for loops, because it means they are guaranteed to end. No for loops are endless!¹¹

Note that there is a conditional statement in the loop above. There is nothing that stops you from putting conditions in the code block for a loop. There is also nothing against putting loops in the code block for a condition, or even putting loops inside loops (more on that last option follows later in this chapter). Most readers probably are not surprised to hear that, but for the few who are completely new to programming: as long as you stick to the syntactic requirements, you can use conditional statements and loops wherever you can write Python statements.

for loop using a range of numbers

Python offers a range() function that generates a collection of sequential numbers, which is often used for for loops. The simplest call to range() has one parameter, which is a number. It will generate all integers, starting at zero, up to but not including the parameter.

for x in range( 10 ):
    print( x )

range() can get multiple parameters. If you give two parameters, then the first will be the starting number (default is zero), while the second will be the “up to but not including” number. If you give three parameters, the third will be a step size (default is 1). You can choose a negative step size if you want to count down. With a negative step size, make sure that the starting number is higher than the number that you want to count up to (or down to, in this case).

for x in range( 1, 11, 2 ):
    print( x )

Change the three parameters above to observe their effect, until you fully understand the range() function.

Use the for loop and range() function to print multiples of 3, starting at 21, counting down to 3, in just two lines of code.

for loop with manual collections

If you want to use a for loop to cycle through items in a collection that you create manually, you can do so by listing all your items between parentheses. This defines a “tuple” for the items of your collection. Tuples will be discussed in detail in Chapter 12.

for x in ( 10, 100, 1000, 10000 ):
    print( x )

Or:

for x in ("apple", "pear", "orange", "banana", "mango", "cherry"):
    print( x )

Your collection can even consist of mixed types.

Practice with for loops

To get strong grips on how to use for loops, do the following exercises.

You already created code with a while loop that asked the user for five numbers, and displayed their total. Create code for this task, but now use a for loop.

Create a countdown function that starts at a certain count, and counts down to zero. Instead of zero, print “Blast off!” Use a for loop.

I am not going to ask you to build a for loop that asks the user to enter numbers until the user enters zero. Why not?