Simple syntax error in Python if else dict compreh

问题:

I have a set and dictionary and a value = 5

v = s = {'a', 'b', 'c'}
d = {'b':5 //<--new value}

If the key 'b' in dictionary d for example is in set s then I want to make that value equal to the new value when I return a dict comprehension or 0 if the key in set s is not in the dictionary d. So this is my code to do it where s['b'] = 5 and my new dictionary is is ...

{'a':0, 'b':5, 'c':0}

I wrote a dict comprehension

{  k:d[k] if k in d else k:0 for k in s}
                          ^
SyntaxError: invalid syntax

Why?! Im so furious it doesnt work. This is how you do if else in python isnt it??

So sorry everyone. For those who visited this page I originally put { k:d[k] if k in v else k:0 for k in v} and s['b'] = 5 was just a representation that the new dictionary i created would have a key 'b' equaling 5, but it isnt correct cus you cant iterate a set like that.

So to reiterate v and s are equal. They just mean vector and set.

回答1:

The expanded form of what you're trying to achieve is

a = {}
for k in v:
    a[k] = d[k] if k in d else 0

where d[k] if k in d else 0 is the Python's version of ternary operator. See? You need to drop k: from the right part of the expression:

{k: d[k] if k in d else 0 for k in v} # ≡ {k: (d[k] if k in d else 0) for k in v}

You can write it concisely like

a = {k: d.get(k, 0) for k in d}

回答2:

In [82]: s = {'a', 'b', 'c'}

In [83]: d = {'b':5 }

In [85]: {key: d.get(key, 0) for key in s}
Out[85]: {'a': 0, 'b': 5, 'c': 0}

回答3:

This should solve your problem:

>>> dict((k, d.get(k, 0)) for k in s)
{'a': 0, 'c': 0, 'b': 5}

回答4:

You can't use a ternary if expression for a name:value pair, because a name:value pair isn't a value.

You can use an if expression for the value or key separately, which seems to be exactly what you want here:

{k: (d[k] if k in v else 0) for k in v}

However, this is kind of silly. You're doing for k in v, so every k is in v by definition. Maybe you wanted this:

{k: (d[k] if k in v else 0) for k in d}