a = [1,2,3,4,5]
b = [6,7,8,9,10]
.
for x in a and b:
print(x)
Output: 6 7 8 9 10
for x in a or b:
print(x)
Output: 1 2 3 4 5
Could someone please explain why this output is produced in these two instances.
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What is the behaviour you're actually trying to get?jonrsharpe– jonrsharpe2014-05-23 14:59:39 +00:00Commented May 23, 2014 at 14:59
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5 Answers
Simple:
>>> a and b
[6, 7, 8, 9, 10]
>>> a or b
[1, 2, 3, 4, 5]
the and operator will return b since it has to check both a and b. The or operator sees a as a value which isn't False thus immediately returning it.
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In Python, a and b evaluates the truthiness of the operands a and b and returns either the first operand that evaluates False or, if both operands evaluate True, the second operand:
>>> [1, 2, 3] and []
[]
>>> [] and [1, 2, 3]
[]
>>> [] and ""
[]
>>> [1, 2, 3] and [4, 5, 6]
[4, 5, 6]
a or b returns the first operand that evaluates True or, if both operands evaluate False, the second operand:
>>> [1, 2, 3] or []
[1, 2, 3]
>>> [] or [1, 2, 3]
[1, 2, 3]
>>> [] or ""
''
>>> [1, 2, 3] or [4, 5, 6]
[1, 2, 3]
They are both lazy, giving up as soon as it can be determined for sure what the outcome will be. Note that empty containers [] and "" evaluate False, whereas non-empy containers like [1, 2, 3] evaluate True.
Comments
>>> a or b
[1, 2, 3, 4, 5]
By definition, a or b evaluates the truthiness of a and, if a is true, returns it (otherwise it returns b).
Comments
The or operator returns as soon as it finds the first True value.
>>> x = 1 or 2
>>> x
1
The and operator evaluates all the operands (in this case two operands) and returns only if all of them are True
>>> x = 1 and 6
>>> x
6
The for loop at each step assigns the comparison result to x. Hence in the first loop it gets the values of first list and in the second loop gets the values of second list as explained above.
Comments
Python's and returns the last truthy (evaluates to True) operand if all are truthy otherwise it returns the first falsey (evaluates to False) operand.
Lists (with length > 0) are truthy regardless of the values in the list so the one to be iterated over depends solely on the oder:
>>> range(1,5) and range(6,10)
[6, 7, 8, 9]
>>> range(6,10) and range(1,5)
[1, 2, 3, 4]
Python's or returns the first truthy operand or the last operand if both are falsey. Again lists (with length > 0) are truthy so it's all about ordering here:
>>> range(1,5) or range(6,10)
[1, 2, 3, 4]
>>> range(6,10) or range(1,5)
[6, 7, 8, 9]
Comments
