A += B Assignment Riddle in Python Last Updated : 16 Jul, 2020 Improve Improve Like Article Like Save Share Report Predict the output of these two expressions on Python console On console Geek = (1, 2, [8, 9]) Geek[2] += [3, 4] Output: Explanation: 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. Like Article Suggest improvement Previous Ways to shuffle a Tuple in Python Next Python | Numpy numpy.ndarray.__imul__() Share your thoughts in the comments Add Your Comment Please Login to comment...