First question about Liao Xuefeng's python Tutorial: Liao Xuefeng's python tutorial

First question about Liao Xuefeng's python Tutorial: Liao Xuefeng's python tutorial

Function parameters are described in the section:

Def add_end (L = []);

L. append ('end ')

Return L

When add_end is called normally (that is, when parameter data transmission is available ):

　　

>>> Add_end ([1, 2, 3]) [1, 2, 3, 'end'] >>> add_end (['x', 'y ', 'Z']) ['x', 'y', 'z', 'end']

When default parameters are used:

>>> add_end()['END']

>>> add_end()['END', 'END']>>> add_end()['END', 'END', 'END']

Liao Xuefeng explained the following:

When defining a Python function, the default parameterLIs calculated, that is[]Because the default parametersLIt is also a variable that points to the object[]. If this function is called every timeLThe content of the default parameter changes next time, instead of when the function is defined.[].

My understanding is as follows:

In the first line, input the parameter [, 3]. The following code is for [, 3]. It has nothing to do with L = []. The third line is the same as the first line, L = [] is not involved in these two times, so the value of L remains unchanged. The second line does not include parameters, so the following code is for L, from [] to ['end'], and the fourth line is for L, however, the value of L has been changed to ['end'] and is run again, so it becomes ['end', 'end'].

The improvement method is as follows:

def add_end(L=None):    if L is None:        L = []    L.append('END')    return L

>>> add_end()['END']>>> add_end()['END']

But I still don't understand the sentence if L is None: L = []. I hope I can understand it later.