Combining two lists with multiple values to a dictionary

Imagine that you have:

keys = ('name', 'age', 'food') values = ('Monty', 42, 'spam') 

What is the simplest way to produce the following dictionary ?

dict = {'name' : 'Monty', 'age' : 42, 'food' : 'spam'} 

This code works, but I'm not really proud of it :

dict = {} junk = map(lambda k, v: dict.update({k: v}), keys, values) 

Replay

Like this:

>>> keys = ['a', 'b', 'c']
>>> values = [1, 2, 3]
>>> dictionary = dict(zip(keys, values))
>>> print dictionary
{'a': 1, 'b': 2, 'c': 3}

Voila :-) The pairwise dict constructor and zip function are awesomely useful: https://docs.python.org/2/library/functions.html#func-dict

Try this:

>>> import itertools
>>> keys = ('name', 'age', 'food')
>>> values = ('Monty', 42, 'spam')
>>> adict = dict(itertools.izip(keys,values))
>>> adict
{'food': 'spam', 'age': 42, 'name': 'Monty'}

It was the simplest solution I could come up with.

PS It's also more economical in memory consumption compared to zip.

>>> keys = ('name', 'age', 'food')
>>> values = ('Monty', 42, 'spam')
>>> dict(zip(keys, values))
{'food': 'spam', 'age': 42, 'name': 'Monty'}

You can also use dictionary comprehensions in Python ≥ 2.7:

>>> keys = ('name', 'age', 'food')
>>> values = ('Monty', 42, 'spam')
>>> {k: v for k, v in zip(keys, values)}
{'food': 'spam', 'age': 42, 'name': 'Monty'}

If you need to transform keys or values before creating a dictionary then a generator expression could be used. Example:

>>> adict = dict((str(k), v) for k, v in zip(['a', 1, 'b'], [2, 'c', 3]))

Take a look Code Like a Pythonista: Idiomatic Python.

with Python 3.x, goes for dict comprehensions

keys = ('name', 'age', 'food')
values = ('Monty', 42, 'spam')

dic = {k:v for k,v in zip(keys, values)}

print(dic)

More on dict comprehensions here, an example is there:

>>> print {i : chr(65+i) for i in range(4)}
    {0 : 'A', 1 : 'B', 2 : 'C', 3 : 'D'}

For those who need simple code and aren’t familiar with zip:

List1 = ['This', 'is', 'a', 'list']
List2 = ['Put', 'this', 'into', 'dictionary']

This can be done by one line of code:

d = {List1[n]: List2[n] for n in range(len(List1))}

Imagine that you have:

keys = ('name', 'age', 'food')
values = ('Monty', 42, 'spam')

What is the simplest way to produce the following dictionary ?

dict = {'name' : 'Monty', 'age' : 42, 'food' : 'spam'}

Python 2

I see some answers mentioning to use izip from itertools, but this goes away in Python 3. However, izip is the best approach for Python 2:

from itertools import izip
new_dict = dict(izip(keys, values))

Python 3

In Python 3, zip becomes the same function that was in the itertools module, so that is simply:

new_dict = dict(zip(keys, values))

Python 2.7 and 3, dict comprehension:

A possible improvement on using the dict constructor is to use the native syntax of a dict comprehension (not a list comprehension, as others have mistakenly put it):

new_dict = {k: v for k, v in zip(keys, values)}

In all cases:

>>> new_dict
{'age': 42, 'name': 'Monty', 'food': 'spam'}

Explanation:

If we look at the help on dict we see that it takes a variety of forms of arguments:

>>> help(dict)

class dict(object)
 |  dict() -> new empty dictionary
 |  dict(mapping) -> new dictionary initialized from a mapping object's
 |      (key, value) pairs
 |  dict(iterable) -> new dictionary initialized as if via:
 |      d = {}
 |      for k, v in iterable:
 |          d[k] = v
 |  dict(**kwargs) -> new dictionary initialized with the name=value pairs
 |      in the keyword argument list.  For example:  dict(one=1, two=2)

The optimal approach is to use an iterable while avoiding creating unnecessary data structures. In Python 2, zip creates an unnecessary list:

>>> zip(keys, values)
[('name', 'Monty'), ('age', 42), ('food', 'spam')]

In Python 3, the equivalent would be:

>>> list(zip(keys, values))
[('name', 'Monty'), ('age', 42), ('food', 'spam')]

and zip merely creates an iterable object:

>>> zip(keys, values)
<zip object at 0x7f0e2ad029c8>

So since we want to avoid creating unnecessary data structures, we usually want to avoid Python 2's zip.



This is a generator expression being passed to the dict constructor:

generator_expression = ((k, v) for k, v in zip(keys, values))
dict(generator_expression)

or equivalently:

dict((k, v) for k, v in zip(keys, values))

And this is a list comprehension being passed to the dict constructor:

dict([(k, v) for k, v in zip(keys, values)])

In the first two cases, an extra layer of non-operative (thus unnecessary) computation is placed over the zip iterable, and in the case of the list comprehension, an extra list is unnecessarily created. I would expect all of them to be less performant, and certainly not more-so.

Performance review:

In 64 bit Python 3.4.3, on Ubuntu 14.04, ordered from fastest to slowest:

>>> min(timeit.repeat(lambda: {k: v for k, v in zip(keys, values)}))
0.7836067057214677
>>> min(timeit.repeat(lambda: dict(zip(keys, values))))
1.0321204089559615
>>> min(timeit.repeat(lambda: {keys[i]: values[i] for i in range(len(keys))}))
1.0714934510178864
>>> min(timeit.repeat(lambda: dict([(k, v) for k, v in zip(keys, values)])))
1.6110592018812895
>>> min(timeit.repeat(lambda: dict((k, v) for k, v in zip(keys, values))))
1.7361853648908436

A more natural way is to use list comprehension

keys = ('name', 'age', 'food')
values = ('Monty', 42, 'spam')
dict = {keys[i]: values[i] for i in range(len(keys))}

Category: python Time: 2008-10-16 Views: 2

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