iterating over shared keys in two dictionaries

Question

I have two dicts. They share some keys. I'd like to iterate over those shared keys. Here are three ways to do it.

x={1:'a', 2:'b', 3:'c'}
y={2:'beta', 3:'gamma', 4:'delta'}

for n in (key for key in x if key in y):
    print(n)

for n in x:
    if n in y:
        print(n)

for n in x:
    if n not in y:
        continue
    print(n)

The first one looks like it will be hard to read in the future.

The second will have lots of indenting in the loop.

The third uses continue which I find to make code a bit harder to follow.

Is there a standard for what I should be using? And is it one of these three?

Answer 1

The best approach depends on what version of Python you're using. They all involve using the & operator for set intersection.

• 3.0+ dict.keys() approach
• 2.7+ dict.viewkeys() approach
• 2.3+ set approach

dict.keys() -- 3.0+

In Python 3, dict.keys() returns a dict_keys object which overrides the & operator[1]:

x={1:'a', 2:'b', 3:'c'}
y={2:'beta', 3:'gamma', 4:'delta'}

for k in x.keys() & y.keys():
    print(k, x[k], y[k])

Output:

2 b beta
3 c gamma

But in Python 2, this will give you a TypeError:

Traceback (most recent call last):
    for k in x.keys() & y.keys():
TypeError: unsupported operand type(s) for &: 'list' and 'list'

dict.viewkeys() -- 2.7+

Don't fret, however! This functionality was backported to Python in 2.7, but under a different name (so as not to break existing code), dict.viewkeys():

for k in x.viewkeys() & y.viewkeys():
    print k, x[k], y[k]

(Same output)

With sets -- 2.3+

Since 2.3 you're able to use sets, and the intersection (&) operator.

s & t: new set with elements common to s and t

For example:

for k in set(x) & set(y):
    print k, x[k], y[k]

(Same output)

Here, however, you're duplicating the space that the keys take up (once in the dictionary, serving as keys, and once again in a completely separate set)

[1] PEP-3106 Revamping dict.keys(), .values() and .items()

Answer 2

If you want to avoid indentation (though, really, it doesn't look like much) and want something clearer than the first one, you could use a set intersection:

for k in x.keys() & y.keys():
    print k

& is the intersection operator, making sure you iterate over shared keys only. If you want to iterate over all keys instead, use x.keys() | y.keys()