Python Quirks: Implicit Return


In Python, functions always have to return something.

>>> def foo():
...  pass
...
>>> print(foo())
None

To ensure that this is the case, instructions that are equivalent to a return None statement are appended to the inner-most code block by the bytecode compiler if no return statement is present.

>>> from dis import dis
>>> dis(foo)
 2 0 LOAD_CONST 0 (None)
 2 RETURN_VALUE
Disassembled foo function

Enforcing this requirement on an individual function level instead of inside the evaluation main loop necessitates that all functions follow this protocol. Let's explore what happens when they don't.

📚 Incomprehensive overview of Python bytecode

The bytecode of existing functions can be examined by looking at their __code__.co_code attribute.

>>> foo.__code__.co_code
b'd\x00S\x00'

Each instruction is 2 bytes long (BB) and consists of an opcode and an optional argument value. struct.unpack can be used to decode individual instructions, and a human readable representation can be obtained through dis.opmap (re-export of the undocumented opcode.opmap), which maps the opcode names to their respective numerical values. Swapping key and value subsequently provides a numerical value to text mapping.

from struct import unpack
from dis import opmap reverse_opmap = {v: k for k, v in opmap.items()} def foo(): pass foo_code = foo.__code__.co_code
for pos in range(0, len(foo_code), 2): inst = unpack("BB", foo_code[pos : pos + 2]) print(f"{reverse_opmap[inst[0]]}: {inst[1]}")
Output
LOAD_CONST: 0
RETURN_VALUE: 0

The bytecode section that instructs the VM to return None is 4 bytes long:

  1. LOAD_CONST(0)

    Loads the constant stored at index 0 of __code__.co_consts (which is None in this case) into memory.

  2. RETURN_VALUE(0)

    Returns the previously loaded constant (argument-less).

🤞 Crossing fingers

Stripping out the 4 bytes, replacing the __code__ attribute of the foo function, and executing the resulting function does not crash the VM (surprisingly). Instead, a SystemError is raised and an accompanying error message is printed to stderr.

from types import FunctionType, CodeType def foo(): pass foo_code = foo.__code__
foo.__code__ = CodeType( foo_code.co_argcount, foo_code.co_kwonlyargcount, foo_code.co_nlocals, foo_code.co_stacksize, foo_code.co_flags, foo_code.co_code[:-4], foo_code.co_consts, foo_code.co_names, foo_code.co_varnames, "", foo_code.co_name, 1, b"",
) foo()
Output
XXX lineno: 1, opcode: 0
Traceback (most recent call last): File "/Users/philip/Developer/testing/python-implicit-return.py", line 25, in <module> foo() File "", line 1, in foo
SystemError: unknown opcode

SystemError is a regular runtime error (despite its name), and can therefor be caught with a regular try / except clause.

>>> try:
...  foo()
... except SystemError:
...  pass

The accompanying error message (XXX lineno: 1, opcode: 0) can't be silence from within Python, because the output stream to which it is written is hard-coded to stderr. That doesn't mean that its impossible, though.

python3 <file> 2> >(sed "/^XXX lineno: [^,]*, opcode: [^\n]*/d" >&2)

🏁 Conclusion

Don't do this.