Python assigning multiple variables to same value? list behavior

If you’re coming to Python from a language in the C/Java/etc. family, it may help you to stop thinking about a as a “variable”, and start thinking of it as a “name”.

a, b, and c aren’t different variables with equal values; they’re different names for the same identical value. Variables have types, identities, addresses, and all kinds of stuff like that.

Names don’t have any of that. Values do, of course, and you can have lots of names for the same value.

If you give Notorious B.I.G. a hot dog,* Biggie Smalls and Chris Wallace have a hot dog. If you change the first element of a to 1, the first elements of b and c are 1.

If you want to know if two names are naming the same object, use the is operator:

>>> a=b=c=[0,3,5]
>>> a is b
True

You then ask:

what is different from this?

d=e=f=3
e=4
print('f:',f)
print('e:',e)

Here, you’re rebinding the name e to the value 4. That doesn’t affect the names d and f in any way.

In your previous version, you were assigning to a[0], not to a. So, from the point of view of a[0], you’re rebinding a[0], but from the point of view of a, you’re changing it in-place.

You can use the id function, which gives you some unique number representing the identity of an object, to see exactly which object is which even when is can’t help:

>>> a=b=c=[0,3,5]
>>> id(a)
4473392520
>>> id(b)
4473392520
>>> id(a[0])
4297261120
>>> id(b[0])
4297261120

>>> a[0] = 1
>>> id(a)
4473392520
>>> id(b)
4473392520
>>> id(a[0])
4297261216
>>> id(b[0])
4297261216

Notice that a[0] has changed from 4297261120 to 4297261216—it’s now a name for a different value. And b[0] is also now a name for that same new value. That’s because a and b are still naming the same object.

Under the covers, a[0]=1 is actually calling a method on the list object. (It’s equivalent to a.__setitem__(0, 1).) So, it’s not really rebinding anything at all. It’s like calling my_object.set_something(1). Sure, likely the object is rebinding an instance attribute in order to implement this method, but that’s not what’s important; what’s important is that you’re not assigning anything, you’re just mutating the object. And it’s the same with a[0]=1.

user570826 asked:

What if we have, a = b = c = 10

That’s exactly the same situation as a = b = c = [1, 2, 3]: you have three names for the same value.

But in this case, the value is an int, and ints are immutable. In either case, you can rebind a to a different value (e.g., a = "Now I'm a string!"), but the won’t affect the original value, which b and c will still be names for. The difference is that with a list, you can change the value [1, 2, 3] into [1, 2, 3, 4] by doing, e.g., a.append(4); since that’s actually changing the value that b and c are names for, b will now b [1, 2, 3, 4]. There’s no way to change the value 10 into anything else. 10 is 10 forever, just like Claudia the vampire is 5 forever (at least until she’s replaced by Kirsten Dunst).

* Warning: Do not give Notorious B.I.G. a hot dog. Gangsta rap zombies should never be fed after midnight.

Related Posts:

  1. Difference between del, remove, and pop on lists
  2. Python – TypeError: ‘int’ object is not iterable
  3. TypeError: list indices must be integers or slices, not str
  4. IndexError: list index out of range and python
  5. Python – TypeError: ‘int’ object is not iterable
  6. Finding the index of an item in a list
  7. Accessing the index in ‘for’ loops?
  8. How to make a flat list out of a list of lists
  9. Removing duplicates in lists
  10. What does the list() function do in Python?
  11. Why does this iterative list-growing code give IndexError: list assignment index out of range?
  12. Is there a short contains function for lists?
  13. Transpose/Unzip Function (inverse of zip)?
  14. appending list but error ‘NoneType’ object has no attribute ‘append’
  15. Check if something is (not) in a list in Python
  16. Python: finding an element in a list [duplicate]
  17. How to avoid “RuntimeError: dictionary changed size during iteration” error?
  18. Writing a list to a file with Python
  19. How do I concatenate two lists in Python?
  20. if else in a list comprehension
  21. How can I randomly select an item from a list?
  22. Get unique values from a list in python [duplicate]
  23. Fastest way to check if a value exists in a list
  24. How can I count the occurrences of a list item?
  25. TypeError: ‘list’ object cannot be interpreted as an integer
  26. if else in a list comprehension
  27. Python set to list
  28. How to find all occurrences of an element in a list
  29. Get list from pandas dataframe column or row?
  30. Finding and replacing elements in a list
  31. How to find all the indexes of a recurring item in a list?
  32. ValueError: max() arg is an empty sequence
  33. Remove list from list in Python
  34. python – if not in list
  35. How do I concatenate two lists in Python?
  36. How can I compare two lists in python and return matches
  37. Replace values in list using Python
  38. Append integer to beginning of list in Python
  39. List comprehension on a nested list?
  40. Union of two lists in Python
  41. Python: Array v. List
  42. Get unique values from a list in python
  43. ‘list’ object has no attribute ‘shape’
  44. How do I check if a list is empty?
  45. Python: Get the first character of the first string in a list?
  47. How do I find the duplicates in a list and create another list with them?
  48. What’s the idiomatic syntax for prepending to a short python list?
  49. TypeError: unsupported operand type(s) for -: ‘list’ and ‘list’
  50. Remove all occurrences of a value from a list?
  51. Converting Dictionary to List?
  52. Python Sets vs Lists
  53. AttributeError: ‘list’ object has no attribute ‘replace’ when trying to remove character
  54. How to deep copy a list?
  55. Python return list from function
  56. How to check if a string is a substring of items in a list of strings?
  57. Whats the difference between ‘for x in list:’ and ‘for x in list[:]:’
  58. Removing Punctuation From Python List Items
  59. How do I check if a list is empty?
  60. IndexError: list index out of range and python
  61. Pop index out of range
  62. Print list without brackets in a single row
  63. How to append multiple values to a list in Python
  64. How do you split a list into evenly sized chunks?
  65. How to iterate through two lists in parallel?
  66. How to remove \n from a list element?
  67. How to read a text file into a list or an array with Python
  68. Map to List error: Series object not callable
  69. how to create a list of lists
  70. How do I check if there are duplicates in a flat list?
  71. How can I create an array/list of dictionaries in python?
  72. How to sort a list/tuple of lists/tuples by the element at a given index?
  73. Append a tuple to a list – what’s the difference between two ways?
  74. How can I remove all instances of an element from a list in Python?
  75. Create an empty list in Python with certain size
  76. How to multiply all integers inside list
  77. Get the cartesian product of a series of lists?
  78. How to concatenate items in a list to a single string?
  79. What’s the difference between lists enclosed by square brackets and parentheses in Python?
  80. Appending to a list gives ‘int’ object has no attribute ‘append’
  81. Python data structure sort list alphabetically
  82. TypeError: unsupported format string passed to list.__format__
  83. How to remove square brackets from list in Python?
  84. Create nice column output in python
  85. TypeError: only integer arrays with one element can be converted to an index 3
  86. How to create a numpy array of lists?
  87. Logical indexing with lists
  88. How does str(list) work?
  89. creating a reverse method for a python list from scratch
  90. Turn a single number into single digits Python
  91. How to create a list of objects?
  92. In Python, is there an elegant way to print a list in a custom format without explicit looping?
  93. Circular list iterator in Python
  94. How is Python’s List Implemented?
  95. Determine if 2 lists have the same elements, regardless of order? [duplicate]
  96. Building a list inside a list in python
  97. Writing Python lists to columns in csv
  98. Create a list with initial capacity in Python
  99. Python initializing a list of lists
  100. Does Python have an immutable list?

Leave a Comment