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This article mainly introduces the expression I + = x and I = I + x in Python is the same, the article is very detailed, has a certain reference value, interested friends must read it!

Is the Python expression I + = x equivalent to I = I + x? If your answer is yes, then congratulations on being 50% correct. Why is it only half right? According to our general understanding, they are equivalent, and there are no similarities or differences in integer operations, but is it the same for list operations? Take a look at the following two pieces of code:

Code 1

> L1 = range (3)

> L2 = L1

> L2 + = [3]

> L1

[0, 1, 2, 3]

> > L2

[0, 1, 2, 3]

Code 2

> L1 = range (3)

> L2 = L1

> L2 = L2 + [3]

> L1

[0, 1, 2]

> > L2

[0, 1, 2, 3]

The value of L2 in Code 1 and Code 2 is the same, but the value of L1 is different, which means that I + = x and I = I + x are not equivalent, so when is it equivalent and when is it not equivalent?

Before you figure this out, you need to understand two concepts: mutable objects and immutable objects.

There are three common properties of any object in Python: unique identity, type, and value.

Unique identity: used to identify the uniqueness of an object in memory. It will not change after the object is created. The function id () can view the unique identity of the object.

Type: depending on which operations the object supports, different types of objects support different operations, for example, lists can have length attributes, but integers don't. Similarly, once the type of the object is determined, it will not change again, and the function type () can return the type information of the object.

The value of the object is different from the unique identification. Not all the values of the object are immutable. The value of some objects can be changed by some operations. The object whose value can be changed is called mutable, and the object whose value can not be changed is called immutable.

Immutable object (immutable)

For immutable objects, the value is always the value at the beginning of creation, and any action on that object will result in the creation of a new object.

> a = 1

> > id (a)

32574568

> a + = 1

> > id (a)

32574544

The integer "1" is an immutable object, and when initially assigned, a points to integer object 1, but after performing a + = operation on variable a, a points to another integer object 2, but object 1 remains unchanged, and variable an already points to a new object 2. Common immutable objects are: int, tuple, set, str.

Mutable object (mutable)

The value of a variable object can be changed dynamically through some operations, such as a list object, which can be constantly added to the list through the append method. The value of the list is constantly changing. When a variable object is assigned to two variables, they share the same instance object and point to the same memory address. When operating on any one of these variables, it will also affect the other variable.

> x = range (3)

> y = x

> > id (x)

139726103041232

> > id (y)

139726103041232

> > x.append (3)

> > x

[0, 1, 2, 3]

> > y

[0, 1, 2, 3]

> > id (x)

139726103041232

> > id (y)

139726103041232

After performing the append operation, the memory address of the object does not change, x and y still point to the same object, but its value has changed.

After understanding mutable objects and immutable objects, back to the question itself, what's the difference between + = and +?

The + = operation will first attempt to call the object's _ _ iadd__ method. If there is no such method, then try to call the _ _ add__ method to see the difference between the two methods.

The difference between _ _ add__ and _ _ iadd__

The _ _ add__ method takes two parameters and returns their sum, and the values of both parameters do not change.

The _ _ iadd__ method also receives two parameters, but it is an in-place operation, which means that it changes the value of the first parameter, because it requires the object to be mutable, so there is no _ _ iadd__ method for immutable objects.

> hasattr (int,'_ _ iadd__')

False

> hasattr (list,'_ _ iadd__')

True

Obviously, the integer object does not have _ _ iadd__, while the list object provides the _ _ iadd__ method.

> L2 + = [3] # Code 1: modify in place using the value of _ _ iadd__,l2

The + = operation in code 1 calls the _ _ iadd__ method, which modifies the value of the object itself that L2 points to.

> L2 = L2 + [3] # Code 2: call _ _ add__, to create a new list and assign a value to L2

The + operation in code 2 calls the _ _ add__ method, which returns a new object, the original object remains the same, L1 still points to the original object, and L2 already points to a new object.

The above is all the content of the article "is the expression I + = x and I = I + x the same in Python?" Thank you for reading! Hope to share the content to help you, more related knowledge, welcome to follow the industry information channel!

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