Python script strange behavior

Question

[dllopezr]

Hi, community!

I am learning python with a book called "Python for Biologist". An exercise is to write a function that calculates the percentage of a residue in a protein sequence and evaluate it with the assert function, and this is my solution:

from _future_ import division
def Residue_percentage(ProteinSequence, Residue):
    length = len(ProteinSequence)
    Residue = Residue.upper()
    residue_count = ProteinSequence.count(Residue)
    percentage = residue_count * 100 / length
    print(percentage)

assert Residue_percentage("MSRSLLLRFLLFLLLLPPLP", "M") == 5

But I got an Assertion error. If I include return percentage the assertion pass the test.

'Do you know what is happening here?

Thank you so much!

[Joe]

That's not strange behaviour at all ;) that's exactly what's meant to happen.

You may want to spend some more time familiarising yourself with what a python function means and how it's used. In particular, return statements, and if you're feeling a little adventurous, you may also want to look in to yield statements, since you'll likely find both in any standard walkthrough or tutorial on functions.

Answer 1

[Alex Reynolds]

If you don't return a value from a Python function, the result of calling that function will be None, which will not equal 5.

Use return to return a testable value.

To avoid dumb Python backwards-incompatibility bugs created by how integers are handled between Python 2 and 3, explicitly cast the result to an integer with int(), i.e., return int(foo) where foo is the value you want test against another integer.