Get TimeStamper working properly if datetime module is mocked before its instantiation

[moriyoshi]

Summary

Because the reference to either datetime.now() or datetime.utcnow() is retrieved during TimeStamper is being created, it gets into trouble when FreezeGun or any similar mechanism that patches datetime module is applied before the instantiation. This patchs proposes a modification to it so that the invocation to the "now" function will always be done with dereferences from the datetime module object, albeit the slight possible performance penalty.

Checklist

• Added tests for changed code.
• Updated documentation for changed code.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives. Find the appropriate next version in our __init__.py file.
• Documentation in .rst files is written using semantic newlines.
• Changes (and possible deprecations) are documented in the changelog.

[moriyoshi]

• This declaration is not necessary, but I prefer the "Φ" to be obvious.

[hynek]

I guess it's a slight performance regression but function calls are getting cheaper over the past releases. :)

oops ci

[hynek]

Looks like mypy is not happy:

src/structlog/processors.py:372: error: Argument 1 to "strftime" of "datetime" has incompatible type "Optional[str]"; expected "str"

[moriyoshi]

Looks like mypy is not happy:
src/structlog/processors.py:372: error: Argument 1 to "strftime" of "datetime" has incompatible type "Optional[str]"; expected "str"

This looks like a false error because fmt should have been asserted as non-None by the preceding if block and I didn't touch the part.

[hynek]

Looks like mypy is not happy:
src/structlog/processors.py:372: error: Argument 1 to "strftime" of "datetime" has incompatible type "Optional[str]"; expected "str"

This looks like a false error because fmt should have been asserted as non-None by the preceding if block and I didn't touch the part.

You're right, looks like a mypy bug!