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Assignment operator

Python library reference says:

Assignment statements are used to (re)bind names to values and to modify attributes or items of mutable objects.

In short, it works as follows (simple assignment):

1. an expression on the right hand side is evaluated, the corresponding object is created/obtained
2. a name on the left hand side is assigned, or bound, to the r.h.s. object

Things to note:

• a single object can have several names bound to it:

  In [1]: a = [1, 2, 3]
  
  In [2]: b = a
  
  In [3]: a
  Out[3]: [1, 2, 3]
  
  In [4]: b
  Out[4]: [1, 2, 3]
  
  In [5]: a is b
  Out[5]: True
  
  In [6]: b[1] = 'hi!'
  
  In [7]: a
  Out[7]: [1, 'hi!', 3]
  

• By modifying b, we have also modified a! This is because a and b are just labels that point to the same object. If we want to make a copy, we can explicitly ask for one by using the list command:

  In [8]: a = [1, 2, 3]
  
  In [9]: b = list(a)
  
  In [10]: a is b
  Out[10]: False
  
  In [11]: b[1] = 'hi!'
  
  In [12]: a
  Out[12]: [1, 2, 3]
  

• Note that taking slices of lists does perform a copy:

  In [13]: a = [1, 2, 3]
  
  In [14]: b = a[:2]
  
  In [15]: b
  Out[15]: [1, 2]
  
  In [16]: b[0] = 'hi!'
  
  In [17]: b
  Out[17]: ['hi!', 2]
  
  In [18]: a
  Out[18]: [1, 2, 3]
  

• The key concept here is mutable vs. immutable

  • mutable objects can be changed in place.
  • immutable objects cannot be modified once created.

A very good and detailed explanation of the above issues can be found in David M. Beazley’s article Types and Objects in Python.