Improve error message [was: Cannot hash a function with `sys.stderr` in the body]

Summary

When a function has print('sth', file=sys.stderr) in the body I get:

InternalHashError: [Errno 2] No such file or directory: '<stderr>'

While caching the body of eval_models_on_all_data(), Streamlit encountered an object of type _io.TextIOWrapper, which it does not know how to hash.

Steps to reproduce

Code snippet:

@st.cache
def f():
   print('', file=sys.stderr)

Debug info

• Streamlit, version 0.73.1
• Python 3.7.9
• Linux roo 5.8.0-33-generic #36-Ubuntu SMP Wed Dec 9 09:14:40 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

[wjaskowski]

This is fine. I understand - I cannot hash anything with side-effects. I just had some error logging inside the function. Sure, I could refactor that to move the logging outside it. However, I raised this issue since: 1) The error message was not clear to me and I struggled to understand that the sys.stderr is to blame. 2) The error message itself explicitly stated that this might be a bug in streamlit and I should raise an issue 😃

[tconkling]

Closing this because our new caching primitives, @st.experimental_memo and @st.experimental_singleton, don’t have this issue (because they don’t attempt to hash all the symbols inside the cached function). New code should prefer memo/singleton over @st.cache.