The result of the string executed by the built-in function exec is not as expected.

>>>a = {}
>>>b = "c"
>>>a[b] = 2

>>>a
{"c": 2}

execute the above statement normal assignment, instead, use exec to execute the string 

>>>exec(f"a = {{}}")    -sharp
>>>exec(f"b = "c"")    -sharp
>>>exec(f"{a}[{b}] = 2")    -sharpa

>>>a
{}

ask why this happens. Sometimes undefined situations such as NameError will occur when exec executes other strings. Please tell me how to change the above exec statement to match the expected statement

.
Python
Mar.04,2021

I don't really know what the question means. 

>>> exec("a={}")
>>> exec("b='c'")
>>> exec("a[b]=2")

clipboard.png

is analyzed in two blocks,
first block, f-string string formatting
second block, exec function 

exec(f"a = {{}}")

is equivalent to exec ("a = {}"), and an is {}

after execution 
exec(f"b = 'c'")

b is'c'

after execution 
exec(f"{a}[{b}] = 2")

is equivalent to exec ("{} [{c}] = 2"), then two problems arise:
first, if c is not defined, it will be reported to NameError: name 'c' is not defined
. Second, if the added object is {} instead of a br a, it will not change.
should be modified to 

exec(f"a['{b}'] = 2")

is equivalent to exec (f "a ['c'] = 2")

in the last digression, it is not recommended to use exec, especially f-string in exec.

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