How to update multiple dictionary values based on a condition?

Question

I have a dictionary which looks like:

dict = {'A':[1,2], 'B':[0], 'c':[4]}

need it to look like:

dict = {'A':[1,2], 'B':[0,0], 'c':[4,0]}

What I am doing right now:

dict = {x: y+[0] for (x,y) in dict.items() if len(y) < 2}

which generates:

dict = {'B':[0,0], 'c':[4,0]}

any idea how I could avoid eliminating those who do not meet the condition?

Answer 1

You're almost there. Try:

my_dict = {x: y + [0] if len(y) < 2 else y
           for (x,y) in dict.items()}

(as mentioned by jp_data_analysis, avoid naming variables after builtins like dict)

Answer 2

This is one way.

Note: do not name variables after classes, e.g. use d instead of dict.

d = {'A':[1,2], 'B':[0], 'c':[4]}

d = {k: v if len(v)==2 else v+[0] for k, v in d.items()}

# {'A': [1, 2], 'B': [0, 0], 'c': [4, 0]}

Answer 3

You can use dictionary comprehension:

d = {'A':[1,2], 'B':[0], 'c':[4]}
new_d = {a:b+[0] if len(b) == 1 else b for a, b in d.items()}

Also, it is best practice not to assign variables to names shadowing common builtins, such as dict, as you are then overriding the function in the current namespace.

Answer 4

1. Your code is almost correct. Your problem is that you're filtering out any lists bigger than 2. What you need to do instead is simply place them in the new dictionary unchanged. This can be done using the ternary operator. It has the form value1 if condition else value2.

2. Also, if you want a more general way to pad every list in your dictionary to be of equal length, you can use map and max.

Here is your code with the above modifications:

>>> d = {'A':[1, 2], 'B': [0], 'c': [4]}
>>> 
>>> max_len = max(map(len, d.values()))
>>> {k: v + [0] * (max_len - len(v)) if len(v) < max_len else v for k, v in d.items()}
{'A': [1, 2], 'B': [0, 0], 'c': [4, 0]}
>>> 

Answer 5

A generalized way:

d = {'A':[1,2], 'B':[0], 'c':[4]}

m = max(len(v) for v in d.values())
for k, v in d.items():
    if len(v) < m:
        d[k].extend([0 for i in range(m-len(v))])

Answer 6

You were very close, just use update():

d = {'A':[1,2], 'B':[0], 'c':[4]}

d.update({x: y+[0] for (x,y) in d.items() if len(y) < 2})

d
# {'A': [1, 2], 'B': [0, 0], 'c': [4, 0]}

Like others have said, don't use reassign reserved names like dict, it's a one way street down to debugging hell.