List comprehension for string in Python, using if else condition

Question

hidden_word = ""
for c in word:
     hidden_word += c if c in guesses else'-'
return hidden_word

I'm trying to accomplish this in a single line of code using list comprehension but am having troubles getting an if-else condition to work and think i am missing something. basically if word = 'yes' and guesses contains 'e' then the result should be '-e-'. I'm able to appropriately put the letter but am having trouble with the logic to put the '-' if the letter is not in guesses.

Answer 1

I think your code is fine - it gets the point across. Why use a list comprehension (and then convert that back to a string)?

hidden_word = ''.join([c if c in guesses else '-' for c in word])

Is that really better? You could change it to a generator expression, but still...

hidden_word = ''.join(c if c in guesses else '-' for c in word)

EDIT: Testing this with a 1000 character "word" using:

import timeit
setup = """import random
chars = "abcdefghijklmnopqrstuvwxyz"
s = "".join(random.choice(chars) for _ in range(1000))
guesses = "agjoxwz"
"""

t1 = "hidden_word = ''.join([c if c in guesses else '-' for c in s])"
t2 = "hidden_word = ''.join(c if c in guesses else '-' for c in s)"
t3 = """hidden_word = ""
for c in s:
     hidden_word += c if c in guesses else '-'"""

Results:

In [24]: timeit.timeit(setup=setup, stmt=t1)
Out[24]: 100.88796829901968

In [25]: timeit.timeit(setup=setup, stmt=t2)
Out[25]: 147.86355467070305

In [26]: timeit.timeit(setup=setup, stmt=t3)
Out[26]: 247.9441536138757

Whoa. So the list comprehension is in fact must faster (and better than a generator expression).

With only 50 letters per "word", the differences are less pronounced, but the list comprehension still wins:

In [28]: timeit.timeit(setup=setup, stmt=t1)
Out[28]: 5.416419290962722

In [29]: timeit.timeit(setup=setup, stmt=t2)
Out[29]: 7.828715333297168

In [30]: timeit.timeit(setup=setup, stmt=t3)
Out[30]: 7.984714775332918