Or keyword inside a print statement in Python?

Question

How does Python decide the output of this ?

print([1, 2] or ["hello"])

I mean why will always print([2] or ["az"]) output [2] and not ["az"] ?

Answer 1

由于这些列表包含元素，因此它们将求值为True因此Python会打印以True字面量为先的那一个。

Answer 2

There are two things you have to understand here. First:

x or y

If x is truthy, it has the value of x (without even evaluating y , so 23 or launch_nukes() doesn't launch the nukes).

Otherwise, it has the value of y .

Or, as the docs put it:

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

Notice that it uses the word "true" here, not the value True . This is a bit confusing (even more so if you're talking out loud, or typing in ASCII without formatting…), which is why everyone says "truthy".

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So, what do "truthy" and "falsey" mean? ¹

• "x is truthy" does not mean x == True , it means bool(x) == True .
• "x is falsey" does not mean x == False , it means bool(x) == False .

For all builtin and stdlib types:

• False is falsey.
• None is falsey.
• Numeric zero values are falsey.
• Empty containers are falsey.
• Everything else is truthy.

Notice that None and empty containers are falsey, but they're not equal to False .

By convention, third-party types (including types that you define ² ) should follow the same rules. But sometimes there are good reasons not to. (For example, a NumPy array is neither truthy nor falsey. ³ )

This is covered loosely in the same docs section :

In the context of Boolean operations, and also when expressions are used by control flow statements, the following values are interpreted as false: False, None, numeric zero of all types, and empty strings and containers (including strings, tuples, lists, dictionaries, sets and frozensets). All other values are interpreted as true. User-defined objects can customize their truth value by providing a __bool__() method.

The exact details for all builtin types are buried in the standard type hierarchy , which is where you learn things like whether bytes is covered by "strings and containers" (yes) or whether there's anything special about NotImplemented (nope, it's truthy).

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So, let's go through your examples:

[1, 2] or ["hello"]

Since [1, 2] is a non-empty container, it's truthy. So this equals [1, 2] .

[] or ["hello"]

Since [] is an empty container, it's falsey. So this equals ["hello"] .

[] == False

[] may be falsey, but it's not False , or even equal to False . Only numbers equal other numbers, and False is the number 0 in the numeric type bool , ⁴ but [] is not a number. So this is False .

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Just be glad you didn't ask about is . :)

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_{1. Technically, these terms aren't defined, even though everyone, even the core devs, uses them all the time. The official reference defines things in terms of evaluating to true or false as a boolean, and then explains what that means elsewhere.}

_{2. You can control whether your types' values are truthy by defining a __bool__ method—or by defining __len__ . The only things you're allowed to do are return True , return False , or raise an exception; if you try to return anything different, it raises a TypeError . So, everything is either truthy, or falsey, or untestable.}

_{3. If you try to check its truthiness, it will raise an exception. That's because NumPy uses boolean arrays widely—eg, array([1, 2]) < array([2, 1]) is array([True, False]) , and you don't want people writing if array([1, 2]) < array([2, 1]): , since whatever they're expecting it to do probably doesn't make sense.}

_{4. Yes, bool is a numeric type—in fact, a subclass of int . This is a little weird when you're first learning, but it turns out to be useful more often than it's dangerous, so it's not just preserved for historic reasons.}

Answer 3

x or y [or z or z1 or z2 or ...] returns the first Truthy element in sequence, or the last Falsey element if all are Falsey.

x and y [and z and z1 and z2 and ...] returns the first Falsey element in sequence, or the last Truthy element if all are Truthy.

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Python has a notion of Truthiness and Falsiness that is separate from True and False . An empty list is not False , but it is Falsey . bool(something_truthy) == True and bool(something_falsey) == False .

Most things are Truthy, so it's easier to list the Falsey things:

• 0 (note that -1 is Truthy)
• None
• Empty collections ( [] , {} , set() , "" , and etc. Note that non-empty collections containing entirely Falsey elements are still truthy eg [None, None, None, None] )
• False

Everything else is Truthy.

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In your example: [1, 2] or ["hello"] == [1, 2] because the first element, [1, 2 is Truthy (the fact that ["hello"] is also Truthy is irrelevant in this case). Note that [1, 2] and ["hello"] == ["hello"] .