how this for loop works in python

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}

It's not a for-loop, but a (nested) dict comprehension. See e.g.: https://www.python.org/dev/peps/pep-0274/ or https://stackoverflow.com/questions/14507591/python-dictionary-comprehension — user2390182, Jul 21 '18 at 12:09

Marcus.Aurelianus · Accepted Answer · 2018-07-21T12:27:01.243

2

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..

edited Jul 21 '18 at 12:27

answered Jul 21 '18 at 12:10

Marcus.Aurelianus

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thank you. y : 0 here : is equal to assignment operator or what ? – star123 Jul 22 '18 at 10:34
@y : 0 equal to d[y]=0, you can check it out here. https://docs.python.org/3/library/stdtypes.html#mapping-types-dict – Marcus.Aurelianus Jul 22 '18 at 10:44
Consider accept the answer by clicking the check on the left of question, it will remove the this from review queue, save other moderators time and distribute the points around. – Marcus.Aurelianus Jul 22 '18 at 10:47

score 0 · Answer 2 · answered Jul 21 '18 at 13:00

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}}

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.

Sorry for my poor expression.

Thank you. You explained very well – star123 Jul 22 '18 at 10:35 — star123, Jul 22 '18 at 10:35

how this for loop works in python

2 Answers2