The premise of Python string pooling

Foreword

In Python, memory pooling technology is often used to improve its performance, so the question is, under what circumstances will pooling? Let us understand through a few examples.

Preliminary knowledge

Before looking at the example, let us first mention a function id() in python, let us look at the function description:

id(obj,/)
 Return the identity of an object.

 This is guaranteed to be unique among simultaneously existing objects.(CPython uses the object \'s memory address.)

Through the above description, we can know that id() will return the unique identifier of the object, and in CPython will return the memory address, that is to say, if the id value of two objects is the same, it can be said that the two objects are the same. .

example

example 00

a =""
b =""print(id(a),id(b))print(a is b)

Output result:

a = “”
b = “”
print(id(a),id(b))
2114853370544 2114853370544
print(a is b)
True

example 01

a ="a"
b ="a"print(id(a),id(b))print(a is b)

Output result:

a = “a”
b = “a”
print(id(a),id(b))
2114883022608 2114883022608
print(a is b)
True

example 02

a ="magic_string"
b ="magic"+"_"+"string"print(id(a),id(b))print(a is b)

Output result:

a = “magic_string”
b = “magic” + “_” + “string”
print(id(a),id(b))
2114887161136 2114887161136
print(a is b)
True

example 03

a ="magic!"
b ="mgaic!"print(id(a),id(b))print(a is b)

Output result:

a = “magic!”
b = “mgaic!”
print(id(a),id(b))
2114885855416 2114889455408
print(a is b)
False

example 04

a,b ="magic!","magic!"print(id(a),id(b))print(a is b)

Output result:
a,b = “magic!”,”magic!”
print(id(a),id(b))
2114885691912 2114885691912
print(a is b)
True

example 05

a ="!"
b ="!"print(id(a),id(b))print(a is b)

Output result:

a = “!”
b = “!”
print(id(a),id(b))
140564571922024 140564571922024
print(a is b)
True

example 06

print(a*20 is 'aaaaaaaaaaaaaaaaaaaa')print(a*21 is 'aaaaaaaaaaaaaaaaaaaaa')

Output result:

print(a20 is ‘aaaaaaaaaaaaaaaaaaaa’)
False
print(a21 is ‘aaaaaaaaaaaaaaaaaaaaa’)
False

to sum up

Through the above 7 examples, it is not difficult for us to have a general understanding of python string pooling. Let's make a brief summary here:

1. Through example 00, 01, 05, we can conclude that strings with a length of 0 or 1 will be pooled
2. Through example 02, 03, we can conclude that strings containing only alphanumeric characters and underscores will be pooled
3. Through example 04, we can get that when assigning different variables on the same line, if they are assigned the same value, they will point to the same object. Note that the "magic!" inside does not meet the requirements of pooling. This is just a compilation Optimization
4. The phenomenon of example 06 has a professional term in python. Constant folding. As the name implies, when compiling and optimizing, the variables that can be calculated are directly replaced with constants. But is there no restriction? Obviously not. In our example, we have found that when the length exceeds 20, the folding will fail. Just imagine, if there is no limit, the initialized string is too long, which will seriously cause performance degradation and memory Consume

Reference link

The internals of Python string interning

exploring python code objects

Python string interning

Python String objects implementation

The above is the detailed content of the premise of Python string pooling. For more information about Python string pooling, please pay attention to other related articles on ZaLou.Cn!