Checkpoint 25.6.1.
What does the code sample above do?
- Produces an error message
- Try running it!
- Prints "That is a valid color." then "Done"
- Correct
- Prints "Done"
- Try running it!
Section 25.6 Complex Conditional Dangers
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 its 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 |
| True | ||
The only way to get the logic we want, is to make sure both the left and right sides 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:
Warning 25.6.3.
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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