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Do we really need to say that getsource(object) can only get the object's source if it is accessible from the object? Getsource also fails if a module is loaded from a .pyc with not corresponding .py available.
The problem is not the call being in __main__. When I put the three lines (with the 3rd wrapped with print()) in an IDLE editor and run, and re-inspect, I get
======================== RESTART: F:\Python\a\tem3.py ========================
class A:
 pass
>>> inspect.getsource(A)
'class A:\n pass\n'
Ditto if I run > py -i -m a.tem3
If I continue in IDLE's Shell
>>> class B: pass
>>> inspect.getsource(B)
Traceback (most recent call last):
 File "<pyshell#3>", line 1, in <module>
 inspect.getsource(B)
 File "F:\dev37円\lib\inspect.py", line 973, in getsource
 lines, lnum = getsourcelines(object)
 File "F:\dev37円\lib\inspect.py", line 955, in getsourcelines
 lines, lnum = findsource(object)
 File "F:\dev37円\lib\inspect.py", line 812, in findsource
 raise OSError('could not find class definition')
OSError: could not find class definition
If I enter the three lines above in a fress python or IDLEs shell, I get the TypeError above.
IDLE does store interactive inputs into linecache, so that tracebacks contain the offending line (unlike interactive python). But it does so on a statement by statement basis, so that each entry is treated as a separate file. In a traceback for an exception in a multiline statement, the line number is relative to the statement.
>>> def f():
	# line2 of f
	1/0
	
>>> f()
Traceback (most recent call last):
 File "<pyshell#13>", line 1, in <module>
 f()
 File "<pyshell#12>", line 3, in f
 1/0
ZeroDivisionError: division by zero
Interactive python displays '<stdin>' as the file for all entries. IDLE numbers them, so previous statements remained cached. I consider enhanced interactive tracebacks to be an important feature.
But I don't see how to attach individual pseudofile names to classes and functions so that getsource could find their source lines.