Backport PR #10788: None The filename was generated taking the last digits of a randomly generated number between 0 and 1, if the number is small enough, it's repr ends with `e-5`, or less (the probability is actually close to 1/1000 because the biggest number we can get with e-5 is (1e-4 - epsilon). We actually did hit that on our CI: - https://ci.appveyor.com/project/IPython/ipython/build/1.0.839/job/mk4l08s295ynkfs5 ``` ====================================================================== ERROR: test_run_submodule_with_absolute_import (IPython.core.tests.test_run.TestMagicRunWithPackage) ---------------------------------------------------------------------- Traceback (most recent call last): File "c:\python36-x64\lib\site-packages\IPython\core\tests\test_run.py", line 455, in test_run_submodule_with_absolute_import self.check_run_submodule('absolute') File "c:\python36-x64\lib\site-packages\IPython\core\tests\test_run.py", line 450, in check_run_submodule self.assertEqual(_ip.user_ns['x'], self.value, KeyError: 'x' -------------------- >> begin captured stdout << --------------------- File "C:\Users\appveyor\AppData\Local\Temp\1\tmp4asmvv33\tmpqpswwe9n\tmp065832886952393e-05\absolute.py", line 2 from tmp065832886952393e-05.sub import x ^ SyntaxError: invalid syntax ``` Actually pick a random string now we should have less chance of being invalid identifier.