Python String Formatting - passing both a dict and string to .format()

Michael Torrie torriem at gmail.com
Wed Nov 27 01:21:15 CET 2013


On 11/26/2013 05:01 PM, Victor Hooi wrote:
> Hi,
> 
> I'm trying to use Python's new style string formatting with a dict
> and string together.
> 
> For example, I have the following dict and string variable:
> 
> my_dict = { 'cat': 'ernie', 'dog': 'spot' } foo = 'lorem ipsum'
> 
> If I want to just use the dict, it all works fine:
> 
> '{cat} and {dog}'.format(**my_dict) 'ernie and spot'
> 
> (I'm also curious how the above ** works in this case).
> 
> However, if I try to combine them:
> 
> '{cat} and {dog}, {}'.format(**my_dict, foo) ... SyntaxError: invalid
> syntax

This is a syntax error because of the way that the ** unpacks the
dictionary.  For this not to be a syntax error, foo has to be before
my_dict.  This is because in parameter passing, keyword args are always
passed last.  In general I don't think you want to unpack the dictionary
in this case.

> I also tried with:
> 
> '{0['cat']} {1} {0['dog']}'.format(my_dict, foo) ... SyntaxError:
> invalid syntax

This is a syntax error because the cat and dog are not valid python
keywords.  A string is anything between two delimiters. So your command
looks to python like this:

'{0[' cat ']} {1} {0[' dog ']}'.format(my_dict, foo)

If you have a proper syntax-highlighting editor you'll see right away
that cat and dog are not within the string delimiters.

This would work, however:

"{0['cat']} {1} {0['dog']}".format(my_dict, foo)

> However, I found that if I take out the single quotes around the keys
> it then works:
> 
> '{0[cat]} {1} {0[dog]}'.format(my_dict, foo) "ernie lorem ipsum
> spot"
> 
> I'm curious - why does this work? Why don't the dictionary keys need
> quotes around them, like when you normally access a dict's elements?

I suppose it's because the string formatter simply doesn't require it.

> Also, is this the best practice to pass both a dict and string to
> .format()? Or is there another way that avoids needing to use
> positional indices? ({0}, {1} etc.)

Can't you just list them as separate arguments to format?  Like you did
in your working example?




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