shadowing and shadowed symbol get mixed in symtable

Question

Consider the following function definition, where the exception name e shadows a parameter:

def f(a,e):
  try:
    raise RuntimeError('test')
  except RuntimeError as e:
    pass

In the symbol table of function f I would expect three symbols: two parameters a,e (not assigned) and one local e, which has been assigned. But the symbol table contains two symbols: parameter a, which has not been assigned, and parameter e, which has been assigned. It seems that the properties of the unassigned parameter e and the exception name e are combined into one symbol. Does that make sense?

See the following test code:

import symtable

def symStat(sym):
  stat = []
  if sym.is_parameter(): stat.append('param')
  if sym.is_assigned(): stat.append('assigned')
  if sym.is_referenced(): stat.append('referenced')
  return stat

INDENT_STR = '  '
def printSymsRec(st, indentLevel=0):
  indent = indentLevel*INDENT_STR
  print('%s%s:' % (indent, st.get_name()),
        [(sym.get_name(), symStat(sym)) for sym in st.get_symbols()])
  for stc in st.get_children():
    printSymsRec(stc, indentLevel+1)

TEST_CODE = """
def f(a,e):
  try:
    raise RuntimeError('test')
  except RuntimeError as e:
    pass
""".strip()

symRoot = symtable.symtable(TEST_CODE, "<string>", "exec")
printSymsRec(symRoot)

which gives the following output:

top: [('f', ['assigned'])]
  f: [('a', ['param']), ('e', ['param', 'assigned']), ('RuntimeError', ['referenced'])]

Why is the output not like the following:

...
  f: [('a', ['param']), ('e', ['param']), ('e', ['assigned']), ...]

@Stephen Not a duplicate. In that case OP thinks the `while` loop creates a scope. — wjandrea, Feb 09 '20 at 19:26
There is only one `e` symbol there. Parameters are local variables; there's no separate namespace for parameters. — user2357112, Feb 09 '20 at 19:26

wjandrea · Accepted Answer · 2020-02-09T19:40:19.510

0

Parameters aren't special in this regard, they're just like any other name. So when except RuntimeError as e assigns to e, it overwrites the parameter's value. And this isn't specific to except statements; you can get the same behaviour from a simple assignment:

TEST_CODE = "def f(e): e = 0"
symRoot = symtable.symtable(TEST_CODE, "<string>", "exec")
printSymsRec(symRoot.get_children()[0])
# -> f: [('e', ['param', 'assigned'])]

edited Feb 09 '20 at 19:40

answered Feb 09 '20 at 19:27

wjandrea

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Ok, I get it: `except ... as e` is - when looking at the name `e` - equivalent to `e = ...`. So the exception name `e` does not shadow the parameter, it remains a parameter. And additionally it gets a value assigned. – coproc Feb 09 '20 at 19:36
@coproc Yeah, exactly. I just added an example about that too. – wjandrea Feb 09 '20 at 19:42

shadowing and shadowed symbol get mixed in symtable

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