Python列表中的布尔值比较给出了奇怪的结果

Question

I try:

[True,True,False] and [True,True,True]

and get [True, True True]

but

[True,True,True] and [True,True,False]

gives

[True,True,False]

Not too sure why it's giving those strange results, even after taking a look at some other python boolean comparison questions. Integer does the same (replace True -> 1 and False ->0 above and the results are the same). What am I missing? I obviously want

[True,True,False] and [True,True,True]

to evaluate to

[True,True,False]

Answer 1

Any populated list evaluates to True . True and x produces x , the second list.

Answer 2

From the Python documentation :

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.

You're getting the second value returned.

PS I had never seen this behavior before either, I had to look it up myself. My naive expectation was that a boolean expression would yield a boolean result.

Answer 3

Others have explained what's going on. Here are some ways to get what you want:

>>> a = [True, True, True]
>>> b = [True, True, False]

Use a listcomp:

>>> [ai and bi for ai,bi in zip(a,b)]
[True, True, False]

Use the and_ function with a map :

>>> from operator import and_
>>> map(and_, a, b)
[True, True, False]

Or my preferred way (although this does require numpy ):

>>> from numpy import array
>>> a = array([True, True, True])
>>> b = array([True, True, False])
>>> a & b
array([ True,  True, False], dtype=bool)
>>> a | b
array([ True,  True,  True], dtype=bool)
>>> a ^ b
array([False, False,  True], dtype=bool)

Answer 4

[True, True, False] is being evaluated as a boolean (because of the and operator), and evaluates to True since it is non-empty. Same with [True, True, True] . The result of either statement is then just whatever is after the and operator.

You could do something like [ai and bi for ai, bi in zip(a, b)] for lists a and b .

Answer 5

As far as I know, you need to zip through the list. Try a list comprehension of this sort:

l1 = [True,True,False]
l2 = [True,True,True]
res = [ x and y for (x,y) in zip(l1, l2)]
print res

Answer 6

and returns the last element if they all are evaluated to True .

>>> 1 and 2 and 3
3

The same is valid for lists, which are evalueted to True if they are not empty (as in your case).

Answer 7

Python works by short-circuiting its boolean and gives the result expression as the result. A populated list evaluates to true and gives the result as the value of the second list. Look at this, when I just interchanged the position of your first and second list.

In [3]: [True,True,True] and [True, True, False]
Out[3]: [True, True, False]