On the change of the value of dictionary append to list in Python

This article mainly introduces about the change of the value of the dictionary append to list in Python, has a certain reference value, now share to everyone, the need for friends can refer to

See an example

d={' test ': 1}d_test=dd_test[' test ']=2print D

If you practice at the command line, you'll notice that you're changing the d_test, but D has changed as well.

Usually this is not the same as what we expected.

Why?

Because dictionary d is an object, D_test=d does not actually create the dictionary in memory again. It just points to the same object. This is also a consideration for Python to improve performance and optimize memory.

Actual scenario

d={"name": "}l=[]for I in Xrange (5):  d[" name "]=i  l.append (d) Print L

Loops may not be the same as the results you want.

Even though the append is in the list, the list contains an object, or a dictionary address. Rather than the real storage space in memory.

Use the. Copy () method. Can create a new stand-alone dictionary

d={"name": "}l=[]for I in Xrange (5):  test=d.copy ()  test[" name "]=i  l.append (test) print L

Update:

a={' Q ': 1, ' W ': []}b=a.copy () b[' Q ']=2b[' W '].append (123) print Aprint b

At this point, the value of ' Q ' in a does not change, but the value in the list has changed.

Because copy is a shallow-level copy

But there's a track here.

a={' Q ': 1, ' W ': []}b=a.copy () b[' Q ']=2b[' W ']=[123]print aprint b

If you assign a value directly, it will not change the structure in a (mostly the relationship of the Append method).

In-depth copy

Import copya={' Q ': 1, ' W ': []}b=copy.deepcopy (a)