What does += mean in Python?

by

a += b is essentially the same as a = a + b, except that:

  • + always returns a newly allocated object, but += should (but doesn’t have to) modify the object in-place if it’s mutable (e.g. list or dict, but int and str are immutable).

  • In a = a + b, a is evaluated twice.

  • Python: Simple Statements

    • A simple statement is comprised within a single logical line.

If this is the first time you encounter the += operator, you may wonder why it matters that it may modify the object in-place instead of building a new one. Here is an example:

# two variables referring to the same list
>>> list1 = []
>>> list2 = list1

# += modifies the object pointed to by list1 and list2
>>> list1 += [0]
>>> list1, list2
([0], [0])

# + creates a new, independent object
>>> list1 = []
>>> list2 = list1
>>> list1 = list1 + [0]
>>> list1, list2
([0], [])

More Related Contents:

  1. Can someone please tell me why the two python codes below produce entirely different outcomes?
  2. What does __all__ mean in Python?
  3. How can I do a line break (line continuation) in Python?
  4. What does the “at” (@) symbol do in Python?
  5. What is the syntax rule for having trailing commas in tuple definitions?
  6. How to use a dot “.” to access members of dictionary?
  7. Putting a simple if-then-else statement on one line [duplicate]
  8. Can you add new statements to Python’s syntax?
  9. `from … import` vs `import .` [duplicate]
  10. Python: access class property from string [duplicate]
  11. “x not in y” or “not x in y”
  12. Python integer incrementing with ++ [duplicate]
  13. Reversing a list using slice notation
  14. Nested list comprehensions
  15. Nested dictionary comprehension python
  16. Python – is there a “don’t care” symbol for tuple assignments?
  17. How do I abort the execution of a Python script? [duplicate]
  18. Python boolean expression and or
  19. Python: format string with custom delimiters [duplicate]
  20. Why does adding a trailing comma after an expression create a tuple?
  21. How to include a quote in a raw Python string
  22. Is it possible to assign the same value to multiple keys in a dict object at once?
  23. Is there a difference between “pass” and “continue” in a for loop in Python?
  24. What do the * (star) and ** (double star) operators mean in a function call?
  25. One try block with multiple excepts
  26. What do >> and
  27. Get the first element of each tuple in a list in Python [duplicate]
  28. Python: SyntaxError: keyword can’t be an expression
  29. Python Syntax Errors with Python Anywhere
  30. Why does newList = list.remove(x) return None? [duplicate]

Leave a Comment