2

I accidentally discovered this a=b=c=d=e=f=2 in python(2.7)(and JavaScript a few minutes later) interpreter .

Is this a feature or just the way the interpreter works, if is a feature how it is called ?

Do other languages have this feature ?

  • Please include code example. – Jack Stone Sep 20 '12 at 15:26
  • a=b=c=d=e=f=2 is the code example you just run it in the interpreter either in python or browser console. – Eduard Florinescu Sep 20 '12 at 15:33
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    I can't think of a language that doesn't support chained assignment... It's existed seemingly forever. – Izkata Sep 20 '12 at 18:05
  • @Izkata Both Pascal and Basic my starting languages didn't support that, maybe you started with C. – Eduard Florinescu Sep 20 '12 at 18:20
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    In C -- int a, b, c = 1; only sets c to 1. a and b are uninitialized. – MebAlone Sep 24 '12 at 18:26
12

In many languages, the = operator returns the value that was assigned.

The line a=b=c=d=e=f=2 is the same as a = (b = ( c = (d = (e = (f = 2)))))

f=2 returns the value of 2.

Thus, this then reduces to a = (b = ( c = (d = (e = 2)))) and so on.

This is known as chained assignment

4

This type of feature is typically known as "multiple assignment." Many languages have this type of feature.

This type of feature is typically known as "chained assignment." Languages that consider assignment to be expressions have this type of feature.

Multiple assignment typically means something else, e.g. Python:

a, b, c, d, e, f = [2, 2, 2, 2, 2, 2]

This type of syntax structure can be referred to as multiple assignment. In the context of Python, this is also known as unpacking.

The astute programmer might note that this is related to multiple return values:

def f(): return [1, 2, 3]
a, b, c = f()

There are a large number of languages that support chained assignment or multiple assignment in different ways.

C, C++, Java, and C# support the syntax you provided. VB does not support chained assignment (since assignment is a statement and not an expression like in the C family). Go supports multiple assignment similarly to Python, e.g. a swap:

a, b = b, a

I believe Lua supports multiple assignment and multiple return values. It can be done in a Lisp but there's usually a better way to go about it.

Basically, most languages support either of these features in some way.

  • I guess a bit of Pascal and Basic still lingers in my mind! – Eduard Florinescu Sep 20 '12 at 15:55
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    Actually, the version in the question is typically called "chained assignment". – Izkata Sep 20 '12 at 18:03
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    multiple assignment (what's described here) is very different from what's being asked. The original question is about "assignment is an expression", which allows another assignment to use the same value. – Javier Sep 20 '12 at 19:19
  • You're right. I made appropriate changes to the answer. – Joel Sep 24 '12 at 17:51
2

Assignments like these are OK for simple objects. For others, it can lead to undesirable side effects. In linked list the example below, in the line:

last = last.nxt = Item(2)

We may expect that Item(1).nxt points to Item(2) and last points to Item(2) However, we end up with a circular reference where Item(2).nxt points to itself.

For example:

class Item(object):
    def __init__(self, value):
        self.value = value
        self.nxt = None

    def __str__(self):
        return 'self: id(self): %d, value: %s, nxt: %d' % (id(self), \
                str(self.value), id(self.nxt))


 prev = last = Item(1)

 print prev
 >>> self: id(self): 140382915976272, value: 1, nxt: 9568656
 print last
 >>> self: id(self): 140382915976272, value: 1, nxt: 9568656
 print id(None)
 >>> 9568656

 last = last.nxt = Item(2)
 print prev
 >>> self: id(self): 140382915976272, value: 1, nxt: 9568656
 print last
 >>> self: id(self): 140382915975120, value: 2, nxt: 140382915975120
  • I wanted to point out the subtle differences between different languages for statements that seemingly look similar. For example, the same type of assignment in C or Java sets up the linked list as expected. ie, item(1).nxt points to item(2) and item(2).nxt points to null. – user136093 Jun 10 '14 at 19:29
1

It is a feature of the language, the = statement evaluates the statement to its right and assigns that value to the variable on the left.

In Python a statement like a, b = 1,2 is also legal. That is called multiple assignment. See Python Docs I don't know of a special term for the a=b=2 structure though. It is just multiple assignments on the same line.

  • 1
    This is wrong. This only works in languages where assignment is not a statement. If it were a statement, then it wouldn't have a value and couldn't be assigned. – Jörg W Mittag Sep 21 '12 at 2:39
  • sorry what in particular is wrong? – Ben McCormick Sep 21 '12 at 16:17
  • @ben336 I believe you have confused statement with expression. if(foo) {} and return 5; are statatements. On the other hand a + 5 and i++ are expressions. In pascal, a := 5; is a statement. In C, a = 5; is an expression. Its semantics, but it is important semantics when talking about languages. – user40980 Sep 24 '12 at 19:12

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