Python for Programmers: Isinstance

Welcome to the Isinstance lesson!

This lesson is shown as static text below. However, it's designed to be used interactively. Click the button below to start!

Suppose that we have a mixed list of integers and strings. We want to sum up all of the integers in the list, ignoring the strings.

donation_amounts = [400, "Amir", 200, None, 100, "Keanu"]

Ideally we'd avoid messy data like this. But sometimes it's unavoidable due to constraints outside of our control. For example, the data might come from another company's API, or from a public data source like a government.
To get the sum, we'll examine each list element and ask Python: "is this an int?" Python has a way to do this: isinstance(value, some_type) returns True when value has that type.
>
isinstance(10, int)
Result:
True
>
isinstance("Hello", int)
Result:
False
>
isinstance("Hello", str)
Result:
True
Remember, floats and integers are not the same type of value. 10.5 is a float, not an int.
>
isinstance(10.5, int)
Result:
False
>
isinstance(10.5, float)
Result:
True
>
isinstance((1, 2), tuple)
Result:
True

isinstance([1, 2], tuple)

Result:

False

Here's our mixed list example again. Now we can use isinstance to decide which list elements to sum.

donation_amounts = [400, "Amir", 200, None, 100, "Keanu"]

donation_total = 0
for donation in donation_amounts:
  if isinstance(donation, int):
    donation_total += donation
donation_total

Result:

What if we want to ask "is this either an int or a float?" We could write isinstance(value, int) or isinstance(value, float). There's also a shorter way: we can pass multiple types in a tuple, like isinstance(value, (int, float)). This means "is value any of these types?"

isinstance(4.5, (int, float))

Result:

True

isinstance("4.5", (int, float))

Result:

False

isinstance((3, 4.5), (int, float))

Result:

False

isinstance((3, 4.5), (tuple, list))

Result:

True

Here's a code problem:
Write a string_count function to count how many of a list's elements are strings (the str type).
def string_count(the_list):
count = 0
for value in the_list:
if isinstance(value, str):
count += 1
return count
assert string_count([None, 1, 3]) == 0
assert string_count(["flavor", 23, "strawberry"]) == 2
assert string_count(["Amir", 32, "Betty", 10, ["Cindy", 4]]) == 2
Goal:
No errors.
Yours:
No errors.
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