Looping in Python

for Loop Cheat Sheet

Looping in Python doesn’t look like looping in other languages.

If you write JavaScript, Java, or other languages, you might have seen code that looks something like this code, that keeps track of 3 things: the starting index, the condition the loop will run until, and which action to take (in this case, incrementing the variable i by 1) until the condition is met.

for (i = 0; i < 5; i++) {
  text += "The number is " + i + "<br>";
}

In fact, before these languages introduced something called a for each loop, that was also the clunky way you’d loop through items in a sequence.

Looping in Python

Looping in Python is a simpler, cleaner process because the Python language prides itself on readability.

Remember you used the in keyword to test if an item was in a sequence? When combined with the for keyword, in can be used to indicate looping over each item in the sequence. The syntax is: for single_item in items, followed by a colon :, followed by a new line, a level of indentation, and the code you’d like to consider as the body of the loop. That is, the code that’ll run multiple times, until there are no more items in the collection.

Let’s see it in action.

>>> colors = ["Red", "Green", "Blue", "Orange"]
>>> for color in colors:
...     print(f"The color is: {color}")
The color is: Red
The color is: Green
The color is: Blue
The color is: Orange

Looping over a range of numbers

Let’s say we wanted to duplicate the code in the example JavaScript above, that prints out the numbers from 0 to 4.

In order to do this, we’ll need to use a built-in function called range(). The range function in python produces a sequence of integers from an optional and inclusive tart, to a defined and exclusive finish.

In Python2, this function created a list of each number in that sequence. As you can imagine, it was horribly inefficient for large ranges. In Python3, the range() function returns a new optimized data type. It’s great for optimization, but it’s harder for debugging.

If you want to explicitly see what a call to range() produces for debugging purposes, you can pass the result into the list() method to see all the values at once. For example: list(range(5)). Remember that this is inefficient, so use it for testing, not in production code.

If we wanted to loop over all the values from 0 to 4, we’d use the range function like this:

>>> for num in range(5):
...     print(f"The number is: {num}")
...
The number is: 0
The number is: 1
The number is: 2
The number is: 3
The number is: 4

You’ll notice that this call didn’t include the number 5.

What if we wanted the range from 1 to 4, instead of 0 to 4? range() can be called with start and stop parameters, and the range will start from start.

>>> for num in range(1, 5):
...     print(f"The number is: {num}")
...
The number is: 1
The number is: 2
The number is: 3
The number is: 4

You can also pass an a third optional step parameter in. Let’s say I quickly wanted to print out all the even numbers from 2 to 10. I would call range(2, 11, 2). Remember, 2 is where we’re starting, 11 is one higher than where we’re ending (10), and 2 is the step, or the amount to jump between numbers.

>>> for num in range(4, 11, 2):
...     print(f"The number is: {num}")
...
The number is: 2
The number is: 4
The number is: 6
The number is: 8
The number is: 10

What do inclusive and exclusive mean in this context? Exclusive means that the end result will not include that number. If you’d like the numbers from 0 to 4, you would call range(5). Consider 5 to the stopping point. Inclusive means the range will include the number. The start parameter is inclusive, meaning if you’d like the range of numbers from 1 to 4, you’d call range(1, 5).

If you can’t remember how to use range, don’t forget to call help(range) from the command line.

Looping over items with the index using enumerate.

In Python, we avoid writing code like like the JavaScript for loop at the top, but sometimes it’s unavoidable, and we need a way to access the index of the items we’re looping through. To do that we use a special function called enumerate(). The function takes a sequence, like a list, and it returns a list of tuples, containing the index of the item in the sequence, and the sequence itself.

Don’t worry about the list of tuples for now, but remember our tuple unpacking from earlier?

>>> point = (2, 5, 11)
>>> x, y, z = point
>>> x
2
>>> y
5
>>> z
11

Because enumerate() returns a structure that looks like a list of tuples under the hood, we can take advantage of tuple unpacking in the the for loop.

>>> for index, item in enumerate(colors):
...     print(f"Item: {item} is at index: {index}.")
...
Item: Red is at index: 0.
Item: Green is at index: 1.
Item: Blue is at index: 2.
Item: Orange is at index: 3.

Remember, indicies in Python start at zero.

Looping over a dictionary

Now that we know we can use tuple unpacking in a for loop, let’s go over how to loop over a dictionary.

Let’s say we have a dictionary of colors to their hex color code used for HTML in websites.

>>> hex_colors = {
...     "Red": "#FF",
...     "Green": "#008",
...     "Blue": "#0000FF",
... }

Remember, a dictionary is composed of key, value pairs. When we loop over a dictionary with the for item in my_dict syntax, we’ll end up looping over just the keys.

In this example, notice how we’re looping over the wrong thing:

>>> for color in hex_colors:
...     print(f"The value of color is actually: {color}")
...
The value of color is actually: Red
The value of color is actually: Green
The value of color is actually: Blue

If we want to loop over the key, value pairs in a dictionary, we’ll want to call my_dict.items().

We can use tuple unpacking along with the my_dict.items() list to loop over both the keys and the values at the same time.

>>> for color, hex_value in hex_colors.items():
...     print(f"For color {color}, the hex value is: {hex_value}")
...
For color Red, the hex value is: #FF0000
For color Green, the hex value is: #008000
For color Blue, the hex value is: #0000FF
Common Errors

What if you try to loop over key, value pairs, and forget to use my_dict.items()?

>>> for color, hex_value in hex_colors:
...     print(f"For color {color}, the hex value is: {hex_value}")
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack (expected 2)

You’ll see ValueError: too many values to unpack (expected 2) if you forget to call my_dict.items(), and try to loop over what you’d expect to be key, value pairs.

Additional Resources

If you really want to be a pro at looping in a Pythonic way, I recommend watching Raymond Hettinger’s talk - Transforming Code into Beautiful, Idiomatic Python after the course.