Python "for" Loops (Definite Iteration)

Python "for" Loops (Definite Iteration)

by John Sturtz basics python
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Watch Now This tutorial has a related video course created by the Real Python team. Watch it together with the written tutorial to deepen your understanding: For Loops in Python (Definite Iteration)

This tutorial will show you how to perform definite iteration with a Python for loop.

In the previous tutorial in this introductory series, you learned the following:

  • Repetitive execution of the same block of code over and over is referred to as iteration.
  • There are two types of iteration:
    • Definite iteration, in which the number of repetitions is specified explicitly in advance
    • Indefinite iteration, in which the code block executes until some condition is met
  • In Python, indefinite iteration is performed with a while loop.

Here’s what you’ll cover in this tutorial:

  • You’ll start with a comparison of some different paradigms used by programming languages to implement definite iteration.

  • Then you will learn about iterables and iterators, two concepts that form the basis of definite iteration in Python.

  • Finally, you’ll tie it all together and learn about Python’s for loops.

A Survey of Definite Iteration in Programming

Definite iteration loops are frequently referred to as for loops because for is the keyword that is used to introduce them in nearly all programming languages, including Python.

Historically, programming languages have offered a few assorted flavors of for loop. These are briefly described in the following sections.

Numeric Range Loop

The most basic for loop is a simple numeric range statement with start and end values. The exact format varies depending on the language but typically looks something like this:

for i = 1 to 10
    <loop body>

Here, the body of the loop is executed ten times. The variable i assumes the value 1 on the first iteration, 2 on the second, and so on. This sort of for loop is used in the languages BASIC, Algol, and Pascal.

Three-Expression Loop

Another form of for loop popularized by the C programming language contains three parts:

  • An initialization
  • An expression specifying an ending condition
  • An action to be performed at the end of each iteration.

This type of loop has the following form:

for (i = 1; i <= 10; i++)
    <loop body>

This loop is interpreted as follows:

  • Initialize i to 1.
  • Continue looping as long as i <= 10.
  • Increment i by 1 after each loop iteration.

Three-expression for loops are popular because the expressions specified for the three parts can be nearly anything, so this has quite a bit more flexibility than the simpler numeric range form shown above. These for loops are also featured in the C++, Java, PHP, and Perl languages.

Collection-Based or Iterator-Based Loop

This type of loop iterates over a collection of objects, rather than specifying numeric values or conditions:

for i in <collection>
    <loop body>

Each time through the loop, the variable i takes on the value of the next object in <collection>. This type of for loop is arguably the most generalized and abstract. Perl and PHP also support this type of loop, but it is introduced by the keyword foreach instead of for.

The Python for Loop

Of the loop types listed above, Python only implements the last: collection-based iteration. At first blush, that may seem like a raw deal, but rest assured that Python’s implementation of definite iteration is so versatile that you won’t end up feeling cheated!

Shortly, you’ll dig into the guts of Python’s for loop in detail. But for now, let’s start with a quick prototype and example, just to get acquainted.

Python’s for loop looks like this:

for <var> in <iterable>:
    <statement(s)>

<iterable> is a collection of objects—for example, a list or tuple. The <statement(s)> in the loop body are denoted by indentation, as with all Python control structures, and are executed once for each item in <iterable>. The loop variable <var> takes on the value of the next element in <iterable> each time through the loop.

Here is a representative example:

>>>
>>> a = ['foo', 'bar', 'baz']
>>> for i in a:
...     print(i)
...
foo
bar
baz

In this example, <iterable> is the list a, and <var> is the variable i. Each time through the loop, i takes on a successive item in a, so print() displays the values 'foo', 'bar', and 'baz', respectively. A for loop like this is the Pythonic way to process the items in an iterable.

But what exactly is an iterable? Before examining for loops further, it will be beneficial to delve more deeply into what iterables are in Python.