System information (version)

• opencv-contrib-python = 4.1.2.30
• Operating System / Platform =>Ubuntu x64
• Compiler => precompiled pip version

Detailed description

The function DualTVL1OpticalFlow contains parameter lambda which cannot be accessed in Python because lambda is reserved word for nameless functions.

I suggest to place underscore at the beginning of all parameters named after greek laters (e.g. _lambda).