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# inbuilt pdb
python -m pdb <script.py>
import pdb; pdb.set_trace()
import pdb; pdb.post_mortem() # invoke pdb after an exception has occurred ?

# with just iPython
from IPython.core.debugger import Tracer; Tracer()()

# after installing ipdb
import ipdb; ipdb.set_trace()

# after installing pudb
import pudb; pu.db

# tracing execution
# https://docs.python.org/2/library/trace.html
python -m trace --trackcalls <foo.py>