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The bytecode compilers ignores ast.Str and ast.Num nodes:
----------------------------
>>> def func():
... 123
... "test"
... 
>>> import dis; dis.dis(func)
 3 0 LOAD_CONST 0 (None)
 3 RETURN_VALUE
----------------------------
But other ast nodes which are constant are not ignored:
----------------------------
>>> def func2():
... b'bytes'
... (1, 2)
... 
>>> import dis; dis.dis(func2)
 2 0 LOAD_CONST 1 (b'bytes')
 3 POP_TOP
 3 4 LOAD_CONST 4 ((1, 2))
 7 POP_TOP
 8 LOAD_CONST 0 (None)
 11 RETURN_VALUE
----------------------------
I don't understand the point of loading a constant and then unload it (POP_TOP).
Attached patch changes the compiler to not emit LOAD_CONST+POP_TOP anymore.
My patch only affects constants. Example with the patch:
----------------------------
>>> def f():
... x
... 
>>> import dis
>>> dis.dis(f)
 2 0 LOAD_GLOBAL 0 (x)
 3 POP_TOP
 4 LOAD_CONST 0 (None)
 7 RETURN_VALUE
----------------------------
The compiler still emits "LOAD_GLOBAL x" for the instruction "x".
Ignoring the Python instruction "x" would change the Python semantics, because the function would not raise a NameError anymore if x is not defined.
Note: I noticed this inefficient bytecode while working on the issue #26146 (add ast.Constant).