Python: How can I modify dictionary values in a for loop alternating the keys?

Question

I am new to Python (and to programming).

I'd like to modify a dictionary in a for loop by alternating the key of the dictionary. I wrote the following code, which was unsccessful, however:

#coding: utf-8
dict1 = {'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}
dict2 = dict.fromkeys(dict1.values(),[])

for key in dict2:
    if key == 'value1':
        dict2[key].extend(['test1', 'test2'])
    elif key == 'value2':
        dict2[key].extend(['test3', 'test4'])
    elif key == 'value3':
        dict2[key].extend(['test5', 'test6'])

print (dict2['value1'])
print (dict2['value3'])

I expected the results to be:

 ['test5', 'test6']
 ['test1', 'test2']

but I actually got:

 ['test5', 'test6', 'test3', 'test4', 'test1', 'test2']
 ['test5', 'test6', 'test3', 'test4', 'test1', 'test2']

I guess the problem might come from my making the dictionary from another dictionary using "dict.fromkeys", but I couldn't see why it is problematic even if it is the case.

Thanks for your attention. Looking forward to your suggestions.

Answer 1

All values of dict2 are actually the same list instance, since passing [] to dict.fromkeys() only creates one list instance. Try

dict2 = {v: [] for v in dict1.values()}

Answer 2

The problem is you used a mutable object as the value initializer. It is the very same object for each value.

Python2> dict1 = {'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}
Python2> dict2 = dict.fromkeys(dict1.values(),[])
Python2> dict2
{'value1': [], 'value2': [], 'value3': []}
Python2> dict2['value1']
[]
Python2> id(dict2['value1'])
43895336
Python2> id(dict2['value2'])
43895336
Python2> id(dict2['value3'])
43895336

So you are extending the same list.