List comprehensions

In the code you've written so far, you may have noticed some common patterns. One of these looks something like this:

new_list = []
for element in old_list:
    if some_condition:
        new_list.append(element)

Let's revisit the GC content calculation for a specific example:

def calculate_gc_ratio(sequence):
    gcs = []
    for base in sequence:
        if base in ['G', 'C']:
            gcs.append(base)
    return len(gcs)/len(sequence)

Do you see the pattern?

This is the pattern that is coveniently wrapped up in list comprehensions.

Here is the general pattern from above, transformed to a list comprehension.

new_list = [element for element in old_list if some_condition]

It looks a little backwards, so compare this to the first example and try to match up parts. It says to check if each element in old_list meets some condition. If it does, store it in a list, which is being assigned to new_list. Notice that all of this is happening in between the list notation [ ... ].

Here is the gc function that uses a list comprehension:

def calculate_gc_ratio(sequence):
    gcs = [base for base in sequence if base in ['G', 'C']]
    return len(gcs)/len(sequence)

Below are a few examples of the kind of things that are commonly done with list comprehensions.

calculate_gc_ratio is an example of filtering a list to remove values that don't fit a certain condition (in this case, bases that aren't a "G" or "C").

Here's one more example that filters a list of numbers for even numbers.

numbers = list(range(1, 11))
evens = [num for num in numbers if num % 2 == 0]
print(evens)

[2, 4, 6, 8, 10]

With the old pattern, this would look like this:

numbers = list(range(1, 11))
evens = []
for num in numbers:
    if num % 2 == 0:
        evens.append(num)

Instead of (or in combination with) filtering a list, we can perform a tranformation on all the list values.

As example, we can take a list of percentages and transform them into ratios.

percents = [99, 34, 89, 77, 88]
ratios = [percent/100 for percent in percents]
print(ratios)

[0.99, 0.34, 0.89, 0.77, 0.88]

In this case, we simply divided by one hundred, but you could also apply a function.