convert json ipython notebook(.ipynb) to .py file

According to https://ipython.org/ipython-doc/3/notebook/nbconvert.html you are looking for the nbconvert command with the --to script option.

ipython nbconvert notebook.ipynb --to script

From the notebook menu you can save the file directly as a python script. Go to the 'File' option of the menu, then select 'Download as' and there you would see a 'Python (.py)' option.

Another option would be to use nbconvert from the command line:

jupyter nbconvert --to script 'my-notebook.ipynb'

Have a look here.