13.6. Complex Conditional Dangers

A common mistake when trying to use and or or is to write things in the way we would in English or in math class.

In natural (human) languages, if I want to say that the color of a car can be blue or green, I would only mention the car once: “The car can be blue or green.” If you try to do that in Python your program will not work as expected:

Why does that happen? In English, we would think of “color equals” as talking about “blue or green”. But in Python, the or splits color == "blue" from "green". Each of those is evaluated on their own. Does color == "blue"? No that is False. How about "green". Is that True or False???

It turns out that any value that is not the number 0 or the empty string "" is considered True in Python. So green counts as True. Since the value on the left of or is False, and the value on the right is True (according to Python), the final value of the expression is True.

color == "blue" or green
False or True

The only way to get the logic we want, is to make sure both the left and right side of the or are expressions that make sense on their own as logical expressions. We have to repeat the color == `` part so that ``"green" is not evaluated on its own:


The items on both side of an and or or MUST be logical expressions (True/False). You can’t have something that looks like ... or "blue" or ... and 10 or that part will just count as True.

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