Python, why elif keyword? [closed]

问题:

I just started Python programming, and I'm wondering about the elif keyword.

Other programming languages I've used before use else if. Does anyone have an idea why the Python developers added the additional elif keyword?

Why not:

if a:
    print("a")
else if b:
    print("b")
else:
    print("c")

回答1:

Most likely its syntactic sugar. Like the Wend of Visual Basic.

回答2:

Far as I know, it's there to avoid excessive indentation. You could write

if x < 0:
    print 'Negative'
else:
    if x == 0:
        print 'Zero'
    else:
        print 'Positive'

but

if x < 0:
    print 'Negative'
elif x == 0:
    print 'Zero'
else:
    print 'Positive'

is just so much nicer.

---

Thanks to ign for the docs reference:

The keyword elif is short for 'else if', and is useful to avoid excessive indentation.

回答3:

Languages with C-like syntax get else if for free without having to implement it at all.

The reason is that in that syntax control structures simply operate on the next statement, which can be a compound statement enclosed in braces if necessary (e.g. { x += 1; y += 1 }).

This means that once you've implemented if and else, else if just falls out of the grammar of the language naturally for free, with no further implementation effort. To see why, have a look at this:

if (condition) {
    if_body();
} else if (another_condition) {
    else_if_body();
} else {
    else_body();
}

This looks like an if with an else if and an else attached, each applied to a compound statement. But in fact it's not. This is actually two separate if statements, each with exactly one else case; the second if statement is inside the body of the else of the first if statement.

else if { ... } is really parsed as else applied to the next statement, which is an if statement (applied to the compound statement { else_if_body(); }. Then the final else binds to the immediately preceding if, which is the second one.

Here's the same thing written more in line with how it's parsed¹:

if (condition) {
    if_body();
} else {
    if (another_condition) {
        else_if_body();
    } else {
        else_body();
    }
}

But it turns out that if the language did directly implement else if as a first-class option for if statements, it would behave exactly the same as the second independent if statement inside the else of the first! So there's no need to bother implementing else if at all; language implementers get else if for free with this style of syntax, once they've implemented if and else.

Python's syntax doesn't allow this freebie.

Programmers of C-style syntax can think in terms of else if even though the language only has if with exactly zero-or-one else, but only because they can write code like my first example that is formatted in a way that looks different to a human reader than it does to the compiler.

Python, OTOH, uses indentation to indicate block structure, which forces the block structure to look the same to a human reader as it does to the interpreter². Once you've got if and else in Python-style syntax, programmers could still write code that behaves identically to an else-if, by putting a second if statement inside the else of a first. But that comes out looking like this:

if condition:
    if_body()
else:
    if another_condition:
        else_if_body()
    else:
        else_body()

This looks ugly, and is much more complex to think in terms of than an else-if chain once you get more than 1 or 2 else-ifs. So it's worth adding in an explicit language feature to get back the ability to think in terms of else-if. Even though it technically makes the language more complex, it actually makes thinking in terms of the language simpler, so it's good complexity; with a manually constructed chain of nested ifs inside elses the reader has to manually read all the code and verify that every else except the last contains exactly one if statement and nothing else, in order to conclude that the whole sequence is equivalent to a linear chain of conditions checked in order, with some code to execute for the first check that succeeds.

So then. We've seen that languages with C-like syntax might as well go with else if, because they get it for free. That's the reason why that exists. Languages with Python-like syntax have to explicitly do something to get a construct that can be used as an else-if. Why did they choose elif? It's arbitrary; you'd have to actually ask the people who made the decision.

However Python didn't invent elif, it was around in other languages long before Python existed. So I would guess that when they had to implement an explicit else-if construct they simply picked one that programmers were already familiar with.

---

¹ Technically, this is how people who are REALLY serious about always using braces with control structures should write their code. ;)

² You can certainly construct counter-examples to this, but it's the general idea of indentation-based syntax.

回答4:

To avoid brace^H^H^H^H^Helse if war.

In C/C++ where you have an else if, you can structure your code in many different styles:

if (...) {
    ...
} else if (...) {
    ...
}


if (...) {
    ...
} 
else if (...) {
    ...
}

if (...) {
    ...
} else 
if (...) {
    ...
}

// and so on

by having an elif instead, such war would never happen since there is only one way to write an elif. Also, elif is much shorter than else if.

回答5:

That's just the way it is. Javascript uses else if, php uses elseif, perl uses elsif, the C preprocessor and python use elif. None of them are wrong, they just choose slightly different syntax to do the same thing. :D

回答6:

I find them helpful to help differentiate the "else-if"s from the "final else".

回答7:

elif is some sort of replacement for switch in other languages but with more power

for example in C you write

switch (number){
 case 1:
   doA()
   break;
 case 2:
   doB()
   break;
 case N:
   doN()
   break;
 default:
   doSomethingElse()
   break;
}

in Python you write

if number == 1: doA()
elif number == 2: doB()
elif number == N: doC()
else: doSomethingElse()

As you see elif is more powerful since you can put more complex statements than in a switch, plus avoid nesting if/else statements

回答8:

Python inherits this from Perl, where it's called elsif.

In Python's case, else if as two separate constructs like it is in C-like languages would be quite ugly as you'd have to have else: if: with two indenting levels.

It's arguable whether special-casing the two keywords together would be better (so making else if a single construct, like the not in operator.

PL/SQL also has elsif, and the C preprocessor has it spelled elif.