Python dictionary copy () method

The copy () method of the Python dictionary surface is deep copy ah, obviously Independent

1 d = {'a'b': 2}2 c = d.copy ()  3print('d=%s  c=%s' % (d, c))

Code1

Results:

D={' A ': 1, ' B ': 2} c={' A ': 1, ' B ': 2}

Modify d to see if the C change is not.

1 d['a']=32print('d=%s  c=%s' % (d, c))

Code2

Results:

D={' A ': 3, ' B ': 2} c={' A ': 3, ' B ': 2}

Here is still the same

Further changes:

 1  d = { " b  : 2,   a  : 3,    C  : ["   A  ,    C   " ]}   b = D.copy ()  3  print  ( d=%s b=%s   ' % (d, b) 

Code3

1 d['C'] = ['a','C' ' Shallow ' ]2print('d=%s  b=%s' % (d, b))

Code4

Code4 to d[' C ') and found that the results are not the same

D={' A ': 3, ' C ': [' a ', ' C ', ' shallow '], ' B ': 2} b={' A ': 3, ' C ': [' a ', ' C '], ' B ': 2}

However, the modification here is to re-assign the value, if it is directly modified it?

1D = {'b': 2,'a': 3,'C': ['a','C']}2b =d.copy ()3 4d['C'].append (123)5 Print('d=%s b=%s'% (d, b))

CODE5

Results:

d={' B ': 2, ' C ': [' a ', ' C ', 123], ' A ': 3} b={' B ': 2, ' C ': [' a ', ' C ', 123], ' A ': 3}

Found that the two are not completely independent, this is the origin of "shallow copy".

So, here's the question: Why is C not changed when you re-assign to D, and C changes when you modify D directly?

In the final analysis, Python is a mechanism problem because assignment operations are all referenced.

Start D,c reference the same dictionary, and when the D is re-assigned, C also points to the original dictionary, but the D reference has changed, so both are independent.

However, when I modify the value of C directly on the basis of the original dictionary (a list).

At this time d,c still the dictionary, so both have changed.

Python dictionary copy () method