Return Statement Gotcha - a case study to get a deeper understanding of the return statement

Return statements can be a source of tricky bugs in your programs. This article walks you through one such case. In the end, you will get a deeper understanding of how return statement works.

The Problem

You are given the following list of strings:

  • Instagram
  • Docs To Go™ Free Office Suite
  • 爱奇艺PPS -《欢乐颂2》电视剧热播
  • Instachat 😜

These represent app names. The goal is to identify which of these strings are “non-English” names. Here’s how we decide which apps have non-English names:

If any of the characters is a non-English character, then the app’s name is not in English.

As a way to solve this problem, the course author suggested the following function:

def is_english(string):
    for character in string:
        if ord(character) > 127:
            return False
    return True

But Saif tried to solve this problem with the following, slightly different, function:

def saif_is_english(string):
    for character in string:
        if ord(character) > 127:
            return False
        else:
            return True

And he got stuck!

He found that for the names - Instagram and 爱奇艺PPS -《欢乐颂2》电视剧热播 - his function returns accurate results, but for Instachat 😜 and Docs To Go™ Free Office Suite , it doesn’t seem to recognize the special characters and returns True anyway.

What’s happening here??

Refresher on how return statement works

The return statement has the property that whenever the computer finds it, when it is running the function, it will quit the function right then and there and not do anything else.

We can read this from the official documentation :

return leaves the current function call with the expression list (or None ) as return value.

Let’s test this. Below I’m defining a function that starts off immediately with a return statement, then it does some stuff and returns something else.

def a_func(n):
    return pow(n,2)    # returns the square of n
    n=n+1              # reassigns n+1 to n
    return n           # returns n (after reassignment)

Let’s try using it:

>>> print(a_func(1))
1
>>> print(a_func(2))
4
>>> print(a_func(3))
9

So we see it’s always returning to us the square of the input. It ignores everything after the first return statement. Let’s now analyze the problem at hand.

Analyzing the Problem in our Case Study

Now that we know how return statements work, let’s see them in action in the context of this question.

We’ll be analyzing the usage of print(saif_is_english('Instagram')) :

  1. First it enters the function with Instagram as its input.
  2. Then it will initiate a “for loop” over Instagram .
    1. character is assgined the value of the first character in the input string, which means that character is assigned I .
    2. The if condition is evaluated:
      • Since ord(character) is 73 , which is smaller than 127 , the statement ord(character) > 127 is false and so we are sent to the else part of the code.
    3. Once in else , we hit the statement return True and the function is exited right here.

So, as you can see, we ended up only ever looking at the first character of Instagram . We can even use Python Tutor’s Visualizer to see this:

saif_is_english

That the function returns a correct result, is merely a consequence of the fact that the first character is “an English character”.

Let’s now see what happens with the usage of print(is_english('Instagram')) :

is_english

Notice (in the right side of the animation) how we only get to a return statement after iterating over all the characters. Here ’s the link for this visualization.

I leave it to you to do this exercise for the other strings. I hope this clears it up.

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