[alexhutcheson]

Honestly I find Python's value vs. reference semantics fairly confusing, and it's not always obvious whether an operation is going to make a copy or modify an existing instance of an object. Good API design and docstrings mitigate this, but it's one more source of complexity to think about. I run into this much less frequently in C++ code - it's normally pretty clear from parameter and return types whether I'm dealing with a copy or a reference.

This might just be because C++ broke my brain into assuming:

   Object foo = other_object;

is a copy operation, and taking a reference or pointer would require extra characters (e.g. copy by default). Most other languages are the opposite: assignment creates a reference by default, and making a copy would require extra characters (e.g. .copy() in Python or .clone() in Java). That's my biggest mental adjustment when moving back-and-forth between C++ and Python.

[nedbat]

Python itself only has one kind of argument passing (neither by-value or classic C++ by-reference), and nothing is copied. Library functions could make confusing choices, it's true.