CPython and PyPy behave differently with the following code:

$ cat diffs.py
print(str(0) is str(0))
$ python3 diffs.py
False
$ pypy diffs.py
True

Deliberate Flexibility

Semi-inevitable Flexibility

e.g. timing

Undesirable Flexibility

Unknown Flexibility

  • Not considered when designing the language
  • such as memory model (how read/write memory in a concurrent environment)

Further Reading