use python list comprehension to update dictionary value

if you're using dict.update don't assign it to the original variable since it returns None

[a.update(price=0) for a in data if a['price']=='']

without assignment will update the list...

Assignments are statements, and statements are not usable inside list comprehensions. Just use a normal for-loop:

data = ...
for a in data:
    if a['price'] == '':
        a['price'] = 0

And for the sake of completeness, you can also use this abomination (but that doesn't mean you should):

data = ...

[a.__setitem__('price', 0 if a['price'] == '' else a['price']) for a in data]

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