Counting Occurrences Using Comprehension

When we want to count the number of items in an iterable, that meet some condition, we can use comprehension to produce an idiomatic syntax:

# Count the numbers in `range(1000)` that are even and contain the digit `9`:
print (sum(
    1 for x in range(1000) 
    if x % 2 == 0 and
    '9' in str(x)
))
# Out: 95

The basic concept can be summarized as:

1. Iterate over the elements in range(1000).
2. Concatenate all the needed if conditions.
3. Use 1 as expression to return a 1 for each item that meets the conditions.
4. Sum up all the 1s to determine number of items that meet the conditions.

Note: Here we are not collecting the 1s in a list (note the absence of square brackets), but we are passing the ones directly to the sum function that is summing them up. This is called a generator expression, which is similar to a Comprehension.