Thank you all for the great explanation, I still trying to find some good
example to use 'global', In CPython, I found an example use 'global' in
cpython/Lib/zipfile.py
_crctable = None
def _gen_crc(crc):
for j in range(8):
if crc & 1:
crc = (crc >> 1) ^ 0xEDB88320
else:
crc >>= 1
return crc
def _ZipDecrypter(pwd):
key0 = 305419896
key1 = 591751049
key2 = 878082192
global _crctable
if _crctable is None:
_crctable = list(map(_gen_crc, range(256)))
crctable = _crctable
_crctable only been used in the _ZipDecrypter function. IIUC, the code can
be refactored to
def _gen_crc(crc):
...stay the same
def _ZipDecrypter(pwd, _crctable=list(map(_gen_crc, range(256)))):
key0 = 305419896
key1 = 591751049
key2 = 878082192
crctable = _crctable
Which avoid using 'global' keyword. Why we are not doing this? I guess the
reason we use 'global' here because we don't want to create `_crctable =
list(map(_gen_crc, range(256)))` every time when we run '_ZipDecrypter'
function. So we kinda cache _crctable with 'global', am I right?
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