String formatting [str.format()] with a dictionary key which is a str() of a number

Question

Python neophyte here. I was wondering if someone could help with the KeyError I am getting when using a dictionary for string interpolation in str.format.

Answer 1

No. According to the documentation:

Because arg_name is not quote-delimited, it is not possible to specify arbitrary dictionary keys (e.g., the strings '10' or ':-]') within a format string.

So you can't use strings consisting of numbers as dictionary keys in format strings.

Note that your key isn't numeric, and it's not trying to use it as a list index. Your key is a string that happens to contain a digit character. What it's trying to do is use the number 1 (not the string "1") as a dictionary key. It will work if you use the number 1 as your dictionary key (i.e., make your dict {'key1': 'val1', 1: 'val2'}).

Answer 2

If your dictionary keys are strings, a possible workaround is to convert your dictionary to a class:

class ClassFromDict:
    def __init__ (self, dct) :
        for key in dct.keys():
            self.__dict__[key] = dct[key]

dictionary = {'key1': 'val1', '1': 'val2'}
classfromdict = ClassFromDict(dictionary)

string1 = 'Interpolating {0.key1}'.format(classfromdict)
print string1
string2 = 'Interpolating {0.1}'.format(classfromdict)
print string2

Answer 3

I think what you are looking for is:

dictionary = {'key1': 'val1', '1': 'val2'}
string2 = 'Interpolating {key1}'.format(**dictionary)
print string2

Answer 4

You're right - the parser isn't that complex, and can't handle variable subscripts for dictionaries. Your work around is:

string2 = 'Interpolating {somevar}'.format(whateverhere..., somevar=dictionary['1'])