For checking

• Google, of course!

• programiz.com

• Python Reference on iOS.

Courses

• Python 3 tutorial on TutorialsPoint.

• Python Tutorial on W3School.

• Python Tutorials by Corey Schafer on Youtube.

• Python 3 on Codecademy (paid).

• Python fundamentals on PluralSight (paid).

• Jobtensor's docs.

Practice

• Hackerrank.

• Exercism.

• CodeSignal.

• CodeChef.

Others

• Newsletter for python news.

• Time complexity (Big O) of various operators in CPython.