On 16/10/2017 18:46, Michael Torrie wrote:
On 10/16/2017 11:21 AM, bartc wrote:
del x effectively removes it from the namespace so trying to use it on
line 4 generates the same 'undefined' error.
However, the byte-code still needs to be aware of x: at the time when
line 1 is executed, the byte-code for line 3 already exists and it will
need to contain a reference to x.
I'm not sure why you'd think this, as we all know that code referring to
a name involves a dictionary lookup at runtime. So the byte code for
line does exist before x is defined, but it does not contain a
"reference" to x in the manner you are thinking. Instead, all line three
knows is that it must lookup the key "x" (as in string) in the
dictionary of local or global objects.
Yes, the reference can be a string. So the string "x" will always exist
in the byte-code even before the program is run, and even if the bit
that uses x is never executed.
So if you look a file of Python code and count 288 different global
variables, then somehow those 288 have to be identified by name in the
binary code.
The symbol tables for them are filled in as it goes along.
I'm just saying it can be done differently. I think it already is for
local variables, otherwise 'STORE_FAST' would require a lookup each
time. As it is, accessing a local 'x' after 'del x' says 'referenced
before assignment'.
--
bartc
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