Why is a Python dict comprehension ignoring the first element in the dict?

Question

I'm trying to convert to floats the string values (which should have been represented as float originally) in the following dict:

{'a': '1.3', 'b': '4'}

If I try a dict comprehension:

{k:float(v) for v in d.values()}

I end up with just the second item in the dict:

In [191]: {k:float(v) for v in d.values()}
Out[191]: {'b': 4.0}

Why is this?

Answer 1

Use d.items() and also you need to refer key,value separately through variables. Here k refers the key and v refers the value.

{k:float(v) for k,v in d.items()}

Example:

>>> d = {'a': '1.3', 'b': '4'}
>>> {k:float(v) for k,v in d.items()}
{'a': 1.3, 'b': 4.0}

Answer 2

The k value is not changed for every v value, change your code to below:

{k:float(v) for k, v in d.items()}

Answer 3

Python dictionaries iteritems would be more appropriate solution.

{k:float(v) for k, v in d.iteritems()}

Also read dict.items() vs dict.iteritems()

Answer 4

Your k variable contains 'b' for some reason. Therefore your expression

{k:float(v) for v in d.values()}

is actually

{'b': float(v) for v in d.values()}

so the result is:

{'b': 4.0}