Don't understand Python lambda function syntax

Question

I don't understand how this syntax work and can't find documentation on it:

word = '123xyz567'
s = ''.join (c if c.isdigit() else ' 'for c in word)

Line 2 of code will take xyz123 and join together "123 567" into one string by passing in a lambda function to join

However, I am confused as to how this syntax works, normally a python if-else statement goes:

def example():
    for c in word:
        if c.isdigit():
            return c
        else:
            return ' '

Can someone either explain or direct me to documentation that explains to me the syntax structure of the lambda function passed into .join() and how I can use it properly?

https://docs.python.org/3/reference/expressions.html#conditional-expressions — iz_, May 02 '21 at 22:39
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score 5 · Answer 1 · answered May 02 '21 at 22:40

5

That's not a lambda but a generator expression.

https://www.python.org/dev/peps/pep-0289/

Almost like a list comprehension but instead a generator literal.

list = [a for a in [1,2,3]]
generator = (a for a in [1,2,3])
print(list) # [1,2,3]
print(generator) # <generator object <genexpr> at 0x7f8ee3abd6d0>

answered May 02 '21 at 22:40

Işık Kaplan

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2

Note that you shouldn't use `list` as a variable name as it might override the built-ins. – 12944qwerty May 02 '21 at 22:43

score 2 · Answer 2 · answered May 02 '21 at 22:47

The syntax you displayed is not a lambda function, but rather a generator expression. See: Generator expressions vs. list comprehensions

You can read more on lambda here: Why are Python lambdas useful?

The def function you displayed wouldn't work properly with the return statement; use yield so that the function won't jump out at the first character:

def example(word):
    for c in word:
        if c.isdigit():
            yield c
        else:
            yield ' '

More on yield here: What does the "yield" keyword do?

score 2 · Accepted Answer · answered May 02 '21 at 22:49

There are several things goining on here, none of them are lambda.

The first thing is a generetor expresion, which is of the form: (<expresion> for <var> in <iterable>).
In your example <expresion> is c if c.isdigit() else ' ', <var> is c, and iterable is word.
It creates a generator (which is a sort of iterable) to pass to str.join.

The second thing is a ternary operator, which is of the form: <expresion> if <bool-expresion> else <expresion>.
In your example the first <expresion> is c, <bool-expresion> is c.isdigit(), and the second <expresion> is ' '.
It returns the first <expresion> if the <bool-expresion> evaluates to True, else it returns the second <expresion>.

Everything put together, (c if c.isdigit() else ' 'for c in word) is a generator with all the characters of word but with non-digit characters replaced with a single space (each).

Don't understand Python lambda function syntax

3 Answers3