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UnboundLocalError is not returned #1454

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dralley opened this issue Oct 2, 2019 · 3 comments
Closed

UnboundLocalError is not returned #1454

dralley opened this issue Oct 2, 2019 · 3 comments
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C-compat A discrepancy between RustPython and CPython

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@dralley
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dralley commented Oct 2, 2019 

x = 10
def foo():
    x += 1
    print(x)
foo()

Expected

Traceback (most recent call last):
  File "test3.py", line 5, in <module>
    foo()
  File "test3.py", line 3, in foo
    x += 1
UnboundLocalError: local variable 'x' referenced before assignment

Actual

11

This might be related to #1333
@dralley dralley added the C-compat A discrepancy between RustPython and CPython label Oct 2, 2019
@sanxiyn
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sanxiyn commented Oct 11, 2019

In fact, there is no UnboundLocalError defined at all.

@Lynskylate
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Lynskylate commented Oct 19, 2019 •
edited
Loading

In fact this is because the generated bytecode isn't compact for python.

def foo():
    x += 1

dis(foo)
# RustPython bytecode
""""
                 0 LoadName             (x, Free)
                 1 LoadConst            (1)
                 2 BinaryOperation      (Add, true)
                 3 StoreName            (x, Free)
""""
# Python bytecode
"""
            0 LOAD_FAST                0 (x)
              2 LOAD_CONST               1 (1)
              4 INPLACE_ADD
              6 STORE_FAST               0 (x)

"""

RusyPython bytecode Load x from free scope, it means that load variables globally when reading from a local scope fails.
In cpython, it just load variable from locals.

//ceval.c 1329
        case TARGET(LOAD_FAST): {
            PyObject *value = GETLOCAL(oparg);
            if (value == NULL) {
                format_exc_check_arg(tstate, PyExc_UnboundLocalError,
                                     UNBOUNDLOCAL_ERROR_MSG,
                                     PyTuple_GetItem(co->co_varnames, oparg));
                goto error;
            }

@dralley
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dralley commented Dec 6, 2020

This seems to be fixed now

@dralley dralley closed this as completed Dec 6, 2020
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@sanxiyn @dralley @Lynskylate