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?

Please go through the intro tour, the help center and how to ask a good question to see how this site works and to help you improve your current and future questions, which can help you get better answers. "Teach me this language feature" is off-topic for Stack Overflow. Stack Overflow is not intended to replace existing tutorials and documentation. — Prune
– Prune, Commented May 2, 2021 at 22:59

Işık Kaplan · Accepted Answer · 2021-05-02 22:40:14Z

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 2, 2021 at 22:40

Işık Kaplan

3,0422 gold badges16 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

12944qwerty Over a year ago

Note that you shouldn't use list as a variable name as it might override the built-ins.

Roy Cohen · Accepted Answer · 2021-05-02 22:49:40Z

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).

Red · Accepted Answer · 2021-05-02 22:47:05Z

2

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?

answered May 2, 2021 at 22:47

Red

27.7k8 gold badges44 silver badges63 bronze badges

Collectives™ on Stack Overflow

Don't understand Python lambda function syntax

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related