这个for循环在python中如何工作

Question

I'm new to python. Please anyone help me to understand this statement of python. How it will work ?

  {x: {y: 0. for y in myClass.features} for x in myClass.items}

Answer 1

Basically what it do is to create a nested dictionary with all values equal to 0.0

class myClassrino:
    def __init__(self):
        self.features=[1,2,3,4,5]
        self.items=[3,4,5,6]

myClass=myClassrino()
output={x: {y: 0. for y in myClass.features} for x in myClass.items}
print(output)

Output is:

{3: {1: 0.0, 2: 0.0, 3: 0.0, 4: 0.0, 5: 0.0}, 4: {1: 0.0, 2: 0.0, 3: 0.0, 4: 0.0, 5: 0.0}, 5: {1: 0.0, 2: 0.0, 3: 0.0, 4: 0.0, 5: 0.0}, 6: {1: 0.0, 2: 0.0, 3: 0.0, 4: 0.0, 5: 0.0}}

Feel free to post anything you are still unclear..

Answer 2

Just give it a try.

{x: {y: 0. for y in [1,2,3]} for x in ['a','b','c']}

=

{'a': {1: 0.0, 2: 0.0, 3: 0.0}, 'b': {1: 0.0, 2: 0.0, 3: 0.0}, 'c': {1: 0.0, 2: 0.0, 3: 0.0}}

Then ones can have some feeling about it from the output.

---

To be easier, you can decompose it:

{y: 0. for y in [1,2,3]}

=

{1: 0.0, 2: 0.0, 3: 0.0}

after substitution, we have

{x: {1: 0.0, 2: 0.0, 3: 0.0} for x in ['a','b','c']}

final answer =

{'a': {1: 0.0, 2: 0.0, 3: 0.0}, 'b': {1: 0.0, 2: 0.0, 3: 0.0}, 'c': {1: 0.0, 2: 0.0, 3: 0.0}}

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Now you only need to replace

[1,2,3] and ['a','b','c']

to

myClass.features and myClass.items

Both are implicitly declared by defining them.

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Sorry for my poor expression.