Question

I want to format a dictionary for printing (Python 2.7.3), and the dictionary has tuples as keys. With other types of keys I can do

>>> coord = {'latitude': '37.24N', 'longitude': '-115.81W', 'altitude':100}
>>> 'Coordinates: {0[latitude]}, {0[longitude]}'.format(coord)
'Coordinates: 37.24N, -115.81W'

I tried the same but it does not work with tuple keys.

>>> a={(1,1):1.453, (1,2):2.967}
>>> a[1,1]
1.453
>>> 'Values: {0[1,1]}'.format(a)

Traceback (most recent call last):
  File "<pyshell#66>", line 1, in <module>
    'Values: {0[1,1]}'.format(a)
KeyError: '1,1'

Why? How I can refer to tuple keys in formatting string?

FOLLOW UP

It seems we can’t (see answer below). As agf quickly pointed out, Python can’t handle this (hope it will be implemented).
In the meantime, I managed to refer to tuple keys in format string with the following workaround:

my_tuple=(1,1)
b={str(x):a[x] for x in a} # converting tuple keys to string keys
('Values: {0[%s]}'%(str(my_tuple))).format(b) # using the tuple for formatting

Answer 1

Under Format String Syntax, field_name is described (emphasis mine):

The field_name itself begins with an arg_name that is either a number or a keyword. If it’s a number, it refers to a positional argument, and if it’s a keyword, it refers to a named keyword argument. If the numerical arg_names in a format string are 0, 1, 2, … in sequence, they can all be omitted (not just some) and the numbers 0, 1, 2, … will be automatically inserted in that order. 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. The arg_name can be followed by any number of index or attribute expressions. An expression of the form '.name' selects the named attribute using getattr(), while an expression of the form '[index]' does an index lookup using __getitem__().

The grammar describes arg_name as:

arg_name          ::=  [identifier | integer]

where identifier is:

identifier ::=  (letter|"_") (letter | digit | "_")*

So a tuple is not a valid arg_name as it is neither an identifier or an integer, and can’t be an arbitrary dictionary key because string keys aren’t quoted.