Question

>>> x = { 'a' : 'b' , 'c' : 'd' }

>>>'a' and 'c' in x
True

>>>'a' and 'b' in x
False

>>>'b' and 'c' in x
True

If in <dict> checks for keys, how come the last one that looks up b returns true even if there is no such key b?

Answer 1

You want 'b' in x and 'c' in x

You’re misunderstanding how the and operator works (and you’ve got your operator precedence wrong). in has a higher precedence than and, so your expression is parsed as:

if 'b' and ('c' in x):

which is the same as:

if 'c' in x:

because bool('b') is always True since 'b' is a non-empty string.

Here is a table of operator precedence in python

Note that even if and had higher precedence than in, you still wouldn’t be getting what you want because ('b' and 'c') in x would reduce down to 'c' in x since 'b' and 'c' returns 'c'.

One way to rewrite your expression would be:

if all( key in yourdict for key in ('b', 'c') ):

This is overkill for just 2 keys to check, but quickly becomes useful if you have more keys to check.

As a final comment, you’re probably trying to apply operator chaining (which is really neat). However, some operators don’t lend themselves to chaining very well (in is one of them). Expressions like 3 > 10 > 100 > 1000 do work by some strange python black magic. In my experience, relational operators chain nicely (‘<‘,’>’,’==’,'<=’,’>=’) but most of the other operators don’t chain in a way that is intuitive. In general,

a operator b operator c operator ...

is equivalent to:

(a operator b) and (b operator c) and (c operator ...