[Python-ideas] Runtime assertion with no overhead when not active

Eloi Gaudry Eloi.Gaudry at fft.be
Sat May 5 04:04:45 EDT 2018

Hi folks,
I intend to add a runtime assertion feature in python. Before submitting a PEP, I am sending a draft to this mailing list to discuss whether it would make sense (above this message). I have actually been using it for the last 2 years and it has proven to be robust and has achieved its primary goal.
Briefly, the idea is to add a new assert that can be switch on/off depending on some variable/mechanism at runtime. The whole point of this assert is that it should not bring any overhead when off, i.e. by avoiding evaluating the expression enclosed in the runtime assert. It thus relies on Python grammar.
Thanks for your feedback.


This PEP aims at offering a runtime assert functionnality, extending the compiletime assert already available.


There is no runtime assert relying on python grammar available. For diagnostics and/or debugging reasons, it would be valuable to add such a feature.

A runtime assert makes sense when extra checks would be needed in a production environment (where non-debug builds are used).

By extending the current python grammar, it would be possible to limit the overhead induces by those runtime asserts when running in a non "assertive" mode (-ed). The idea here is to avoid evaluating the expression when runtime assertion is not active.

A brief example why avoiding evaluating the expression is needed to avoid any overhead in case the runtime assert should be ignored.
runtime_assert( 999 in { i:None for i in range( 10000000 ) } )

runtime_assert( expr )

#would result in if expr and runtime_assert_active:
print RuntimeAssertionError()
Implementation details

There is already an implementation available, robust enough for production. The implementation is mainly grammar based, and thus the parser and the grammar needs to be updated:

  *   Python/graminit.c
  *   Python/ast.c
  *   Python/symtable.c
  *   Python/compile.c
  *   Python/Python-ast.c
  *   Python/sysmodule.c
  *   Modules/parsermodule.c
  *   Include/Python-ast.h
  *   Include/graminit.h
  *   Include/pydebug.h
  *   Grammar/Grammar
  *   Parser/Python.asdl
  *   Lib/lib2to3/Grammar.txt
  *   Lib/compiler/ast.py
  *   Lib/compiler/pycodegen.py
  *   Lib/compiler/transformer.py
  *   Lib/symbol.py
  *   Modules/parsermodule.c


PEP 306, How to Change Python's Grammar (http://www.python.org/dev/peps/pep-0306)


This document has been placed in the public domain.

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