Assigning variable value to another in Python [duplicate]

Question

This is hard for me to understand:

import numpy
f=numpy.array([1,2])
g=f
g[0]=f[0]+1
print(f)

The output of this code is [2,2]. How can I change the value of g without changing f?

Your question assumes that f and g are different, but they are just two different names for the same object. Python's = creates names. — Klaus D.
– Klaus D., Commented Jan 30, 2019 at 4:51
Ned batchelder wrote a really blog about this: Facts and myths about Python names and values. One important point there is "Many names can refer to one value." and "Assignment never copies data" — MSeifert
– MSeifert, Commented Jan 30, 2019 at 5:00
@dyukha, FYI if you try to make a new question using the title I created: the question referenced does not show up in the results. — J. Ramos
– J. Ramos, Commented Jan 30, 2019 at 5:46
@dyukha easy to say if you are an expert/already know the answer to what are you looking for, right?. As another comentator placed it,this is a very common question, so my advice would be to be more empathic the next time. — J. Ramos
– J. Ramos, Commented Jan 30, 2019 at 6:03
@J.Ramos, Being common doesn't make it any good. It means that lots of people don't do research/minimize enough etc. You should just formulate your problem clearly enough. What you have done: you created a copy (at least you think so). What happens: when you change a copy, your original changes. Now you should place this into one sentence and google (with some keywords, like numpy). No "expert" skills required. — user2956272
– user2956272, Commented Jan 30, 2019 at 6:11

Jose Angel Sanchez · Accepted Answer · 2019-01-30 04:42:35Z

1

This is because the way pointer work, you need to copy the variable not make a reference to it

In [16]: import copy
In [17]: import numpy
    ...: f=numpy.array([1,2])
    ...: g=copy.deepcopy(f)
    ...: g[0]=f[0]+1
    ...: print(f)
    ...:
    ...:
[1 2]

In [18]: f
Out[18]: array([1, 2])

In [19]: g
Out[19]: array([2, 2])

answered Jan 30, 2019 at 4:42

Jose Angel Sanchez

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