Behaviour of increment and decrement operators in Python

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I notice that a pre-increment/decrement operator can be applied on a variable (like ++count). It compiles, but it does not actually change the value of the variable!

What is the behavior of the pre-increment/decrement operators (++/--) in Python?

Why does Python deviate from the behavior of these operators seen in C/C++?

Behaviour of increment and decrement operators in Python, ++ is not an operator. It is two + operators. The + operator is the identity operator, which does nothing. (Clarification: the + and - unary operators only work on  >>> a = 0 >>> >>> #Increment >>> a +=1 >>> >>> #Decrement >>> a -= 1 >>> >>> #value of a >>> a 0 Python does not provide multiple ways to do the same thing . However, be careful if you are coming from a languae like C, Python doesn’t have “variables” in the sense that C does, instead python uses names and objects and in python integers(int’s) are immutable.

When you want to increment or decrement, you typically want to do that on an integer. Like so:

b++

But in Python, integers are immutable. That is you can't change them. This is because the integer objects can be used under several names. Try this:

>>> b = 5
>>> a = 5
>>> id(a)
162334512
>>> id(b)
162334512
>>> a is b
True

a and b above are actually the same object. If you incremented a, you would also increment b. That's not what you want. So you have to reassign. Like this:

b = b + 1

Or simpler:

b += 1

Which will reassign b to b+1. That is not an increment operator, because it does not increment b, it reassigns it.

In short: Python behaves differently here, because it is not C, and is not a low level wrapper around machine code, but a high-level dynamic language, where increments don't make sense, and also are not as necessary as in C, where you use them every time you have a loop, for example.

Behaviour of increment and decrement operators in Python, Behaviour of increment and decrement operators in Python. I notice that a pre-increment/decrement operator can be applied on a variable (like ++count). It compiles, but it does not actually change the value of the variable! Why does Python deviate from the behavior of these operators seen in C/C++? Increment and Decrement Operators in Python. If you’re familiar with Python, you would have known Increment and Decrement operators ( both pre and post) are not allowed in it. Python is designed to be consistent and readable. One common error by a novice programmer in languages with ++ and -- operators is mixing up the differences (both in precedence and in return value) between pre and post increment/decrement operators.

While the others answers are correct in so far as they show what a mere + usually does (namely, leave the number as it is, if it is one), they are incomplete in so far as they don't explain what happens.

To be exact, +x evaluates to x.__pos__() and ++x to x.__pos__().__pos__().

I could imagine a VERY weird class structure (Children, don't do this at home!) like this:

class ValueKeeper(object):
    def __init__(self, value): self.value = value
    def __str__(self): return str(self.value)

class A(ValueKeeper):
    def __pos__(self):
        print 'called A.__pos__'
        return B(self.value - 3)

class B(ValueKeeper):
    def __pos__(self):
        print 'called B.__pos__'
        return A(self.value + 19)

x = A(430)
print x, type(x)
print +x, type(+x)
print ++x, type(++x)
print +++x, type(+++x)

Behaviour of increment and decrement operators in Python, It is true that python does not have any increment or decrement operator like ++​value etc, unlike languages like c++, java etc. This is because in  Other languages have for loops which uses increment and decrement operators. Python offers for loop which has a range function having default increment value “1” set. We can also specify our increment count as a third argument in the range function

Python does not have these operators, but if you really need them you can write a function having the same functionality.

def PreIncrement(name, local={}):
    #Equivalent to ++name
    if name in local:
        local[name]+=1
        return local[name]
    globals()[name]+=1
    return globals()[name]

def PostIncrement(name, local={}):
    #Equivalent to name++
    if name in local:
        local[name]+=1
        return local[name]-1
    globals()[name]+=1
    return globals()[name]-1

Usage:

x = 1
y = PreIncrement('x') #y and x are both 2
a = 1
b = PostIncrement('a') #b is 1 and a is 2

Inside a function you have to add locals() as a second argument if you want to change local variable, otherwise it will try to change global.

x = 1
def test():
    x = 10
    y = PreIncrement('x') #y will be 2, local x will be still 10 and global x will be changed to 2
    z = PreIncrement('x', locals()) #z will be 11, local x will be 11 and global x will be unaltered
test()

Also with these functions you can do:

x = 1
print(PreIncrement('x'))   #print(x+=1) is illegal!

But in my opinion following approach is much clearer:

x = 1
x+=1
print(x)

Decrement operators:

def PreDecrement(name, local={}):
    #Equivalent to --name
    if name in local:
        local[name]-=1
        return local[name]
    globals()[name]-=1
    return globals()[name]

def PostDecrement(name, local={}):
    #Equivalent to name--
    if name in local:
        local[name]-=1
        return local[name]+1
    globals()[name]-=1
    return globals()[name]+1

I used these functions in my module translating javascript to python.

Behaviour of increment and decrement operators in Python, When we try to do increment or decrement, we typically do that on an integer. Like as follows: b++. But when we deal with Python, so we see  Python doesn’t support the various increment/decrement pre- and post-fix operators from C because they were deemed to be a bad idea —- most likely to lead to bugs and difficult to maintain code. Python maintains a distinction between statements (especially assignment or “binding” statements) and expressions.

In Python, a distinction between expressions and statements is rigidly enforced, in contrast to languages such as Common Lisp, Scheme, or Ruby.

Wikipedia

So by introducing such operators, you would break the expression/statement split.

For the same reason you can't write

if x = 0:
  y = 1

as you can in some other languages where such distinction is not preserved.

What is behavior of ++ and -- operators in Python?, It compiles, but it does not actually change the value of the variable! What is the behavior of the pre-increment/decrement operators (++/--) in Python? Why does  Python: Behaviour of increment and decrement operators. Hi, I've tried this. ++num and the num doesn't change at all, always show the value when initialized. if I change ++num to num+=1 then it works. So, my question is how that ++ operator works?

Why is there no increment operator (++) or decrement operator (--) in , In C/C++ and Java etc, ++ and -- operators are defined as increment and decrement operators. In Python they are not defined as operators. It was just designed that way. Increment and decrement operators are just shortcuts for x = x + 1. Python has typically adopted a design strategy which reduces the number of alternative means of performing an operation. Augmented assignment is the closest thing to increment/decrement operators in Python, and they weren't even added until Python 2.0.

Python - Increment and Decrement Operators, I notice that a pre-increment/decrement operator can be applied on a variable (​like ++count ). It compiles, but it does not actually change the  Increment and decrement operators are unary operators that add or subtract one, to or from their operand, respectively. They are commonly implemented in imperative programming languages. C-like languages feature two versions of each operator with slightly different semantics. In languages syntactically derived from B, the increment operator is written as ++ and the decrement operator is written as --. Several other languages use inc and dec functions. The increment operator increases, and the de

Python's Fake Increment and Decrement Operators, Originally Answered: Why doesn't Python allow an increment operator? Well, you Behaviour of increment and decrement operators in Python. It has a very  In this article, you will learn about the increment operator ++ and the decrement operator -- in detail with the help of examples. In programming (Java, C, C++, JavaScript etc.), the increment operator ++ increases the value of a variable by 1. Similarly, the decrement operator -- decreases the value of a variable by 1.

Comments
  • Python is not C or C++. Different design decisions went into making the language. In particular, Python deliberately does not define assignment operators that can be used in an arbitrary expression; rather, there are assignment statements and augmented assignment statements. See reference below.
  • What made you think python had ++ and -- operators?
  • Kaizer: Coming from C/C++, I write ++count and it compiles in Python. So, I thought the language has the operators.
  • @Fox You're assuming a level of planning and organisation not in evidence
  • @mehaase ++ and -- don't exist in c "as syntactic sugar for pointer arithmetic", they exist because many processors have automatic increment and decrement memory access mechanisms (in general pointer indexing, stack indexing) as part of their native instruction set. For instance, in 6809 assembler: sta x++ ...the atomic instruction that results stores the a accumulator where x is pointing, then increments x by the size of the accumulator. This is done because it is faster than pointer arithmetic, because it is very common, and because it's easy to understand. Both pre- and -post.
  • "The + operator is the "identity" operator, which does nothing." Only for numeric types; for other type it is an error by default.
  • Also, be aware that, in Python, += and friends are not operators that can be used in expressions. Rather, in Python they are defined as part of an "augmented assignment statement". This is consistent with the language design decision in Python to not allow assignment ("=") as an operator within arbitrary expressions, unlike what one can do in C. See docs.python.org/reference/…
  • The unary + operator has a use. For decimal.Decimal objects, it rounds to current precision.
  • I'm betting on parser simplification. Note an item in PEP 3099, "Things that will Not Change in Python 3000": "The parser won't be more complex than LL(1). Simple is better than complex. This idea extends to the parser. Restricting Python's grammar to an LL(1) parser is a blessing, not a curse. It puts us in handcuffs that prevent us from going overboard and ending up with funky grammar rules like some other dynamic languages that will go unnamed, such as Perl." I don't see how to disambiguate + + and ++ without breaking LL(1).
  • It's not correct to say that ++ is nothing more than a synonym for += 1. There are pre-increment and post-increment variants of ++ so it is clearly not the same thing. I agree with the rest of your points, though.