how are strings immutable in python?

Terry Reedy tjreedy at udel.edu
Sun Jul 6 22:50:33 CEST 2008



Peter Otten wrote:
> ssecorp wrote:
> 
>>>>> h = "aja baja"
>>>>> h += 'e'
>>>>> h
>> 'aja bajae'
> 
> The inplace-add operator doesn't mutate the lvalue, it just rebinds it:

In Python, neither '=' nor members of the 'op=' family are operators.
They all mark *assignment* or *augmented assignment* statements that 
*all* bind objects to targets.

Augmented assignments are, rather obviously, restricted to binding one 
object to one target.  For 'x op= y', if the object originally bound to 
x is mutable, the arithmetic operation part of the augmented assignment 
can (should) be implemented by an inplace __i<opname>__ special method 
that (normally, but not necessarily) mutates and returns self to be 
rebound.  Otherwise, the interpreter calls the normal __<opname>__ 
special method  (if it exits) that returns a new object to be bound.

Thus, '+=' is neither an operator nor is the indicated operation 
necessarily inplace.

Immutable built-in classes do not have __i<opname>__ methods.  So given 
that name h is bound to a string,
   h += 'e'
has exactly the same effect as
   h = h + 'e'
which has exactly the same effect as
   h = h.__add__('e')
The same is true for immutable instances of other built-in classes.


Terry Jan Reedy




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