I cannot reproduce this behavior in 3.7+. `assert (0, 'bad')` does not have any effect. But in 3.6 it outputs a traceback to the stderr.

$ ./python -m idlelib
Exception in Tkinter callback
Traceback (most recent call last):
  File "/home/serhiy/py/cpython3.6/Lib/tkinter/__init__.py", line 1705, in __call__
    return self.func(*args)
  File "/home/serhiy/py/cpython3.6/Lib/idlelib/multicall.py", line 176, in handler
    r = l[i](event)
  File "/home/serhiy/py/cpython3.6/Lib/idlelib/pyshell.py", line 1205, in enter_callback
    self.runit()
  File "/home/serhiy/py/cpython3.6/Lib/idlelib/pyshell.py", line 1246, in runit
    self.interp.runsource(line)
  File "/home/serhiy/py/cpython3.6/Lib/idlelib/pyshell.py", line 684, in runsource
    return InteractiveInterpreter.runsource(self, source, filename)
  File "/home/serhiy/py/cpython3.6/Lib/code.py", line 64, in runsource
    code = self.compile(source, filename, symbol)
  File "/home/serhiy/py/cpython3.6/Lib/codeop.py", line 168, in __call__
    return _maybe_compile(self.compiler, source, filename, symbol)
  File "/home/serhiy/py/cpython3.6/Lib/codeop.py", line 82, in _maybe_compile
    code = compiler(source, filename, symbol)
  File "/home/serhiy/py/cpython3.6/Lib/codeop.py", line 133, in __call__
    codeob = compile(source, filename, symbol, self.flags, 1)
SyntaxWarning: assertion is always true, perhaps remove parentheses?


But other syntax warnings are converted to errors in IDLE (reported by Raymond in https://bugs.python.org/issue15248#msg349375).

In the regular interactive interpreter:

>>> 0 is 0
<stdin>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
True
>>> data = [
...          (1, 2, 3) # oops, missing comma!
...          (4, 5, 6)
...      ]
<stdin>:2: SyntaxWarning: 'tuple' object is not callable; perhaps you missed a comma?
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
TypeError: 'tuple' object is not callable

In IDLE:

>>> 0 is 0
SyntaxError: "is" with a literal. Did you mean "=="?
>>> data = [
         (1, 2, 3) # oops, missing comma!
         (4, 5, 6)
     ]

SyntaxError: 'tuple' object is not callable; perhaps you missed a comma?