wtform import performances

[azmeuk]

I have been profiling the unit test part of a flask project lately. My issue is that launching a single unit test takes ~4s on a very capable computer. This is not acceptable to me as I do test driven development, each second is multiplied by dozen and dozen runs each day, and this gives me a frustrating sensation. My investigations lead me to look around import times.

I have been profiling my tests with the unfamous importtime python option with something like: python -X importtime -c "import web; web.create_app()".

By deferring some library calls from the conftest.py files root to the inside of my pytest fixtures, I could save some precious seconds. Now the bottleneck has moved, and it seems some of latency is due to wtforms, and among others this import and the ones behind it:

wtforms/src/wtforms/validators.py

Lines 6 to 9 in efe4e90

	try:
	import email_validator
	except ImportError:
	email_validator = None

In addition to that, the wtforms descriptor-like behavior implies that the fields are instantiated at import time the if a form class is defined at the module level. In addition to that, validators objects must be instantiated to be passed to fields:

wtforms/src/wtforms/fields/core.py

Lines 184 to 188 in efe4e90

	if inspect.isclass(validator):
	raise TypeError(
	"{} is not a valid validator because it is a class, "
	"it should be an instance".format(validator)
	)

So if a form class is defined at the module level, all the fields and all the objects validators are instantiated.

In order to slightly improve those performances I suggest to :

• Move the library imports (email_validator, ipaddress etc.) from the module level to inside the validators
• Either:
  • allow fields to take class validators (that should be instantiated a bit later) and not just objects and callable validators
  • transform all the class validators in functions validators. This one is my favorite option.

I would love to hear you on that @davidism