On 3/15/2019 8:47 AM, Arup Rakshit wrote:
Hi,
I am reading a book where it says that:
Just like module-level function definitions, the definition of a local function
happens at run time when the def keyword is executed. Interestingly, this means
that each call to sort_by_last_letter results in a new definition of the
function last_letter. That is, just like any other name bound in a function
body, last_letter is bound separately to a new function each time
sort_by_last_letter is called.
If that above is true, why the below program shows the same object reference
for last_letter every time I call function sort_by_last_letter.
# file name is sample.py
def sort_by_last_letter(strings):
def last_letter(s):
return s[-1]
print(last_letter)
return sorted(strings, key=last_letter)
python3 -i sample.py
sort_by_last_letter(['ghi', 'def', 'abc'])
<function sort_by_last_letter.<locals>.last_letter at 0x1051e0730>
['abc', 'def', 'ghi']
sort_by_last_letter(['ghi', 'def', 'abc'])
<function sort_by_last_letter.<locals>.last_letter at 0x1051e0730>
['abc', 'def', 'ghi']
sort_by_last_letter(['ghi', 'def', 'abckl'])
<function sort_by_last_letter.<locals>.last_letter at 0x1051e0730>
['def', 'ghi', 'abckl']
To build on Calvin's explanation ...
intersperse other function definitions between the repeated calls
sort_by_last_letter(['ghi', 'def', 'abc'])
def a(): return 'skjsjlskjlsjljs'
print(a)
sort_by_last_letter(['ghi', 'def', 'abc'])
def b(): return 546465465454
print(b)
sort_by_last_letter(['ghi', 'def', 'abc'])
and memory gets reused a different way.
<function sort_by_last_letter.<locals>.last_letter at 0x03A51D40>
<function a at 0x03A51D40> # <== is same memory as .....^^^^
<function sort_by_last_letter.<locals>.last_letter at 0x043C2710>
<function b at 0x043C2710> # ditto
<function sort_by_last_letter.<locals>.last_letter at 0x043C2768>
Creating a new list or string did not have the same effect. I believe
that CPython function objects must currently all have the same size or
at least the same max size and conclude that CPython currently allocates
them from a block of memory that is some multiple of that size. These
are, of course, current internal implementation details, subject to
change and even variation across hardware and OSes.
--
Terry Jan Reedy
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