A += B Assignment Riddle in Python
Predict the output of these two expressions on Python console
Look at the bytecode Python generates for the expression s[a] += b. It becomes clear how that happens.
It works step by step
- Put the value of s[a] on Top Of Stack(TOS).
- Perform TOS += b. This succeeds, If TOS refers to a mutable object. That is why appending item to list is successful.
- Assign s[a] = TOS. This fails, If s is immutable. TypeError, because of tuple are immutable in above example.
Things to learn
- Putting mutable items in tuples is not a good idea.
- Augmented assignment is not an atomic operation — we just saw it throwing an exception after doing part of its job.
- Inspecting Python bytecode is not too difficult, and is often helpful to see what is going on under the hood.