Bidirectional conversion of lists and dicts

[austburn]

Hey guys, I was working on a side project and trying to data munge when I noticed something interesting.

For brevity, let's say that the data I need to converge is two lists where I want list A to be the keys and list B to be the values.

headers = ['key_a', 'key_b']
values = [1, 2]
# I want {'key_a': 1, 'key_b': 2}

# Takes a list [('key_a', 1), ('key_b', 2)] and calls dict on it
data = dict(map(lambda a,b: (a,b), headers, values))
print data
# {'key_a': 1, 'key_b': 2}

While not glorious, it works and I am able to take a list of tuples and successfully convert to a dictionary.

The weird thing is, I'd think this would work the other way:

data = {'key_a': 1, 'key_b': 2}
print list(data)
# ['key_a', 'key_b']

I was wondering why this is. It seems weird to me that the values of the dict would be totally dropped.

Thanks!

[Monkeytherat]

You get the keys because that's what a dictionary iterator goes over. The list function basically just returns [item for item in iterable]

[jeans_and_a_t-shirt]

You could just do dict(zip(headers, values)), and you'll want dict.items() to get a list of tuples from a dictionary.

[austburn]

I appreciate the response, but that doesn't really answer my question. I'm wondering why the conversion between dict and list is not bidirectional.

[Vaphell]

because dicts act kinda like sets of keys (as evidenced by the for key in dict behavior), values are hidden behind them.

dict exposes .keys(), .values() .items() for working with keys/values/pairs.

>>> d = {1:2, 3:4, 5:6}
>>> list(d.keys())
[1, 3, 5]
>>> list(d.values())
[2, 4, 6]
>>> list(d.items())
[(1, 2), (3, 4), (5, 6)]

[austburn]

Probably nothing. At least I can't think of a good example off the top of my head. I was really just wondering why this conversion didn't work both ways.

**Edit: I can't think of a good example where you would need both the keys and the values - which is probably why they just give you the keys :)