How can I fix "SyntaxError: invalid syntax" for this list?

Question

Here is a very simple code:

a = [1, 2, 3, 4, 5, 6, 7, 8, 9]
b = 1
c = [b*=i for i in a]
print(c)

I am trying to multiply all of the numbers in the list a, but I get a syntax error for line 3. How can I fix the code?

You can't use assignment operators inside a list comprehension — 3vts, Sep 20 '20 at 00:12
What's your expected output? You might be looking for something like `[(b := b*i) for i in a]` in Python 3.8. — wjandrea, Sep 20 '20 at 00:16
@wjandrea: If they have 3.8, I strongly suspect [they really want `c = math.prod(a)`](https://docs.python.org/3/library/math.html#math.prod). Prior to 3.8, it would be `c = functools.reduce(operator.mul, a, 1)`. Unless they really want a `list` of all the intermediate products *and* the final product (in which case, yes, they want what you suggested). — ShadowRanger, Sep 20 '20 at 00:21

Carcigenicate · Accepted Answer · 2020-09-20T00:26:06.093

= and the augmented versions like it aren't expressions that evaluate to a value. The issue is, that's what list comprehensions are expecting: an expression.

The comprehension consists of a single expression followed by at least one for clause and zero or more for or if clauses.

To do this, you'd need Python 3.8 and assignment expressions:

[b := b * i for i in a]

There is no augmented version that combines * and :=.

Consider just using a full loop though.

Rfroes87 · Answer 2 · 2020-09-20T00:25:53.133

1

Another option for this is the following:

import operator
from functools import reduce

a = [1, 2, 3, 4, 5, 6, 7, 8, 9]

b = reduce(operator.mul, a, 1)

print(b)

By the way, this was referenced in this question:

EDIT: As noted by @ShadowRanger, starting in Python 3.8 you could simplify this to:

import math

a = [1, 2, 3, 4, 5, 6, 7, 8, 9]

b = math.prod(a)

print(b)

edited Sep 20 '20 at 00:25

answered Sep 20 '20 at 00:17

Rfroes87

1

In 3.8+, `math.prod` would do the trick more efficiently/obviously. – ShadowRanger Sep 20 '20 at 00:22

score 0 · Answer 3 · answered Sep 20 '20 at 00:14

0

As mentioned in the comments, you can't do item assignment inside of a list comprehension. What you could do is

b=1
for x in a:
    b*=x
print(b)

answered Sep 20 '20 at 00:14

Daniel Walker

3 Answers3