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.