Source

Girl Develop It San Francisco, Teaching Materials, Exercises: Strings: MixUp

Description

Given two strings s1 and s2 and an integer n, swap the first n characters of s1 and s2 and concatenate the results, separated by a space. If either string is less than or equal to n characters in length, return them unmodified but concatenated, separated by a space.

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'foo', 'bar', 3 ⟹ 'foo bar'
'foobar', 'bazqux', 3 ⟹ 'bazbar fooqux'

My solution

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def headswap(s1, s2, n):
    short = len(s1) <= n or len(s2) <= n
    swap = lambda a, b: f'{a[:n]}{b[n:]}'
    return f'{s1} {s2}' if short else f'{swap(s2, s1)} {swap(s1, s2)}'

Tests

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import pytest
from headswap import *

def test_headswap_nomod():
    """Should leave strings unmodified."""
    assert headswap('foo', 'bar', 3) == 'foo bar'

def test_headswap_mod():
    """Should modify strings."""
    assert headswap('foobar', 'bazqux', 3) == 'bazbar fooqux'