嵌套循环的python字典理解

Question

I am starting to understand and implement dictionary comperhension. I am trying to build a dictionary with nested dictionaries like this:

{serviceA:{ip_server1: flag, ip_server2:flag...}, serviceB{ip_server1: flag, ip_server_2: flag...etc}}

I am having problems to build it as when getting the result It just throws me tha last element. I have tried many ways and all lead the same way. Here is the code I used :

DICTIONARY 1:

services_dict = {key_service: value for key_service,value in config_file.iteritems() if key_service.startswith("s-")}
#It returns a dictionary in this form {service1: [list_of_flags]}

Then I have an external list that has the server IPs

server_list = [S1, S2, S3, S4..etc]

After this I want to use dic comprehension to build the complete dictionary:

new_services_dict = { key1: {key2:fl} for key1, flag_list in corrected_dict.items() for key2, fl in zip(server_list, flag_list)}

The problem is that it only returns the last element of the ip list (1 element only)

{'ServiceA': {'last_server_ip_in_list': 'Last_flag_in_list'}}

I am not sure why it is not appending new values and just take sthe last item in the iteration.

Any help would be greatly appreciated. I have already tried many ways and i am not able to find out which part of the iteraiton i am missing.

UPDATED INFO

As requested in the coment below here is some sample data to clarify the question: Let say we have to map a list of students that I have separated from a dictionary that contains the college subjects with the grades:

services_dict = {math:['A','B','C','D'], Databases:['B','C','D', 'A']} students list = ['Jhon','Michael','Leslie','Lorraine']

The grades are listed respectively from the student list, so that is not the problem (on which grade belongs to whom).

What is intended it to get a dictionary in the form:

{'math':{'Jhon':'A', 'Michael':'B', 'Leslie':'C', 'Lorraine':'D'}, Databases:{'Jhon':'B', 'Michael':'C', 'Leslie':'D', 'Lorraine':'A'}}

The problem i am getting is that new_services_dict is only returning:

{'math:{'Lorraine':'D'}

I hope this is clearer than the explanation above.

Answer 1

Well for starters, it looks like all the keys in new_services_dict will be the same since you have a nested for loop and not a nested dictionary comprehension, which I am assuming is what you want.

For example,

>>>{k1:{k2:v} for k1 in ['a','b'] for k2,v in zip(['aa','bb'],[1,2])}
{'b': {'bb': 2}, 'a': {'bb': 2}} # dictionaries are unordered

Here k1 takes value of 'a' then pauses until k2,v for loop iterates through all of the items before resuming. This just keeps reassigning k1 to {'bb':2} since that's the last item in the k2 iteration. This is why all of your values are probably coming out the same.

What you probably want is a nested dictionary comprehension that would look like this:

>>>{k1:{k2:v for k2,v in zip(['aa','bb'],[1,2])} for k1 in ['a','b']}
{'b': {'aa': 1, 'bb': 2}, 'a': {'aa': 1, 'bb': 2}}

Here, k1 takes the value of 'a' then pauses until the nested comprehension runs.

Check this out for more information and better explanation on nested comprehensions . This is for list comprehensions, but the same logic applies.