Petula Macready

A for loop in Python uses collection-based iteration. This means that Python assigns the next item from an iterable to the loop variable on every iteration, like in this example:

In this example, values is a list with three strings, "a", "b", and "c". In Python, lists are one type of iterable object. In the for loop, the loop variable is value. On each iteration of the loop, value is set to the next item from values.

Next, you print value onto the screen. The advantage of collection-based iteration is that it helps avoid the off-by-one error that is common in other programming languages.

Now imagine that, in addition to the value itself, you want to print the index of the item in the list to the screen on every iteration. One way to approach this task is to create a variable to store the index and update it on each iteration:

In this example, index is an integer that keeps track of how far into the list you are. On each iteration of the loop, you print index as well as value. The last step in the loop is to update the number stored in index by one. A common bug occurs when you forget to update index on each iteration:

In this example, index stays at 0 on every iteration because there’s no code to update its value at the end of the loop. Particularly for long or complicated loops, this kind of bug is notoriously hard to track down.

Another common way to approach this problem is to use range() combined with len() to create an index automatically. This way, you don’t need to remember to update the index:

In this example, len(values) returns the length of values, which is 3. Then range() creates an iterator running from the default starting value of 0 until it reaches len(values) minus one. In this case, index becomes your loop variable. In the loop, you set value equal to the item in values at the current value of index. Finally, you print index and value.

With this example, one common bug that can occur is when you forget to update value at the beginning of each iteration. This is similar to the previous bug of forgetting to update the index. This is one reason that this loop isn’t considered Pythonic.

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