In the direct process of rendering a scene, a Blender camera itself has no distortion coefficients. Distortion is inserted a posteriori as a compositing node, see How to tweak K1, K2 and K3 undistortion values in motion tracking?.
In the inverse process of calibrating cameras from a given video clip, Blender's motion tracking interface estimates distortion parameters K1, K2 and K3, to be used in rendering through compositing in the aforementioned way. To access these distortion coefficients, you have to be in the Movie Clip Editor, load a clip, and press n to toggle on/off region containing the Lens panel:
Notice the option to select the desired type of distortion model ("Polynomial" in this case).
You can access these parameters in Python:
In case you want to render with custom lens distortion parameters, not obtained from Blender's motion tracking, see Custom nonlinear distortion lens