Understanding Chained Assignments in Python

Chained assignments in Python, exemplified by expressions like "x = y = somefunction()", have sparked discussions regarding their equivalence to multiple sequential assignments ("x = somefunction(); y = somefunction()"). To clarify this matter, let's explore the operation of chained assignments in detail.

In a chained assignment, the leftmost target is assigned first. Therefore, "x = y = somefunction()" is equivalent to:

temp = somefunction()
x = temp
y = temp

This order is crucial, as the target to the left receives the value of the temporary variable "temp" before it is assigned to the target to the right. This is evident in the disassembly of a chained assignment function:

>>> def chained_assignment():
...     x = y = some_function()
...
>>> import dis
>>> dis.dis(chained_assignment)
  2           0 LOAD_GLOBAL              0 (some_function)
              3 CALL_FUNCTION            0
              6 DUP_TOP
              7 STORE_FAST               0 (x)
             10 STORE_FAST               1 (y)
             13 LOAD_CONST               0 (None)
             16 RETURN_VALUE

Note that the value returned by the function is stored in "x" first, then in "y."

This understanding is crucial for comprehending the behavior of chained assignments. For instance, expressions like "x = y = []" should be avoided, as both "x" and "y" will refer to the same list. Instead, favor separate assignments like "x = []" and "y = []" to create distinct lists.