Deleting elements from lists in a dictionary, Python

Question

I have a dictionary with certain keys and lists of couples as elements. I want to perform the following operation: remove every element (couple) from all the lists for which its second element (or first, doesn't really matter) fulfills a certain condition. I tried with a very simple piece of code to do that, but I did not succeed and I in fact noticed a specific behavior: removing an element form a list let's the list's elements for loop skip the following element, for some reason (or at least this is what it looks like to me).

This is a very simple example:

# random dictionary
a = {'a': [[1, 1], [1, 2], [1, 3], [1, 4], [1, 5], [1, 6], [1, 7]],
     'b': [[2, 1], [2, 2], [2, 3], [2, 4], [2, 5], [2, 6], [2, 7]]}

def f(d):
    
    # keys in the dicitonary
    for key in list(d):
        
        # elements inthe list
        for elem in d[key]:
            
            # if the second element of the couple
            # satisfies a certain criterion...
            if elem[1] >= 2:
                
                # remove that element (first occurrence) from the list
                d[key].remove(elem)
        
    return d

b = f(a)
print(b)

This is the output I get:

{'a': [[1, 1], [1, 3], [1, 5], [1, 7]], 'b': [[2, 1], [2, 3], [2, 5], [2, 7]]}

This is what I would like to have:

{'a': [[1, 1]], 'b': [[2, 1]]}

How do I perform the above described operation correctly?

Edit: I know there are some workarounds. My question is now about the skipping thing occurring in the for loop: why does it happen? Why does the code not work as I think it should?

Answer 1

You could iterate over the values (which are lists) and in a nested loop iterate over those lists again and if a certain value doesn't satisfy your condition, ignore it.

# random dictionary
a = {'a': [[1, 1], [1, 2], [1, 3], [1, 4], [1, 5], [1, 6], [1, 7]],
     'b': [[2, 1], [2, 2], [2, 3], [2, 4], [2, 5], [2, 6], [2, 7]]}

def f(d):
    # iterate over the keys
    for key in d.keys():
        value_list = d[key]
        # create a temporary list
        temp_list = []
        for elem in value_list:
            # if some element doesn't match the criteria, skip it
            if elem[1] >= 2:
                continue
            # if the condition is satisfied, add it to the temporary list
            temp_list.append(elem)
        # replace the key's value with the modified list
        d[key] = temp_list
    
    return d

b = f(a)
print(b)