CODEWITHSUNDEEP
pythonif-statement
Aeleon
Aeleon

Reputation: 169

What is this piece of code doing, python

I am self learning python and I was doing an exercise, the solution to which was posted in this thread. Could anyone translate into english what this piece of code means? When I learned if statements I never came across this syntax.

 consonants = 'bcdfghjklmnpqrstvwxz'
 return ''.join(l + 'o' + l if l in consonants else l for l in s)

Upvotes: 6

Views: 220

Answers (2)

chepner
chepner

Reputation: 530990

This piece of code contains neither an if statement nor a for loop. You can tell most easily because statements cannot be embedded in an expression, such as the one being passed as the argument to join.

consonants = 'bcdfghjklmnpqrstvwxz'
return ''.join(l + 'o' + l if l in consonants else l for l in s)

The if is part of a conditional expression, which has the form x if y else z. Unlike an if statement, the else is mandatory, because the expression must have a value whether or not y is true. If y is true, the value is x; otherwise, the value is z. In this case, the value is either l + 'o' + l (when l is a consonant) or l itself.

The for keyword is used to indicate a generator expression, which you can think of as essentially as a way to generate a sequence of values based on another sequence. Here, we start with the some sequence s, and produce another value for each of the characters l in that sequence. The resulting sequence is used by join to produce a new string.

(Slightly off-topic, but it's more efficient to explicitly pass a list, not a generator, to join, since it needs to build a list first anyway to figure out how much space to allocate for the resulting string.

return ''.join([l + 'o' + l if l in consonants else l for l in s])

)

Upvotes: 0

Zach Gates
Zach Gates

Reputation: 4130

It's a longer piece of code, written as a generator. Here is what it would look like, more drawn out.

consonants = 'bcdfghjklmnpqrstvwxz'

ls = []
for l in s:
    if l in consonants:
        ls.append(l + 'o' + l)
    else:
        ls.append(l)

return ''.join(ls)

It loops through s and checks if l is in the string consonants. If it is, it pushes l + 'o' + l to the list, and if not, it simply pushes l.

The result is then joined into a string, using ''.join, and returned.

More accurately (as a generator):

consonants = 'bcdfghjklmnpqrstvwxz'

def gencons(s):
    for l in s:
        if l in consonants:
            yield l + 'o' + l
        else:
            yield l

return ''.join(gencons(s))

Where gencons is just a arbitrary name I gave the generator function.

Upvotes: 11

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