Python string concatenated to more than 2, with join instead of + number

Python string connection efficiency issues

The connection efficiency of strings has been mentioned in the discussion of Python efficiency, and most recommend using join instead of "+" for string connections

Everything in Python object string object is the C API stringobject.c see Python Source objects file can be found

1. "+". Using the plus sign to connect 2 strings calls the static function String_concat (register pystringobject *a, register pyobject * b), which in this function will open a block of memory that is the size of a+b and then A, b string Copy in, imagine, if it is n strings connected then will open up n-1 times memory, is more resource-intensive

2. For the use of the Join function "". The join ([A, b]) call to the String_join function calculates the length of the list one time to allocate a chunk of memory and then copy it, so the number of times that n strings are connected to open memory is 1 times.

So if there's only 2 connections, the difference is small, but there's a lot of data.

Python differs from the C language when creating variables

1. Python has a small integer buffer pool pyintobject is an extension of the numeric value of long in C, for performance reasons, for small integers, Python uses a small integer object pool to small_ints cache an integer between [-5,257]. Integers within this range are shared in the Python system. when you define an integer a=3. Then delete this integer using B=3 to discover that the ID of a, B is the same, then use C = 12345 to perform the above operation we found 2 numbers of memory addresses are not the same small integer buffer pool

2. Python defines a string s = "Hello", g = "Hello" after, the address of S,g is the same, C language definition char a[5] = "Hello", char b[5] = "Hello", the string address of A and B is different, indicating that another memory is requested, PY The interned mechanism used by Thon first queries

>>> a= "123"

>>> b= "123"

>>> ID (a)

46553808L

>>> ID (b)

46553808L

>>> a=12345

>>> b=12345

>>> ID (a)

47307928L

>>> ID (b)

47307880L

Small integers:

>>> a=256

>>> b=256

>>> ID (a)

35165744L

>>> ID (b)

35165744L

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Python string joins more than 2, with join instead of + sign