How to detect that Python code is being executed through the debugger?

$ USING_PDB=1 pdb yourprog.py

import os
if os.environ.get('USING_PDB'):
    # debugging actions
    pass

import inspect

def isdebugging():
  for frame in inspect.stack():
    if frame[1].endswith("pydevd.py"):
      return True
  return False

import sys
if 'pydevd' in sys.modules:
    print "Debugger"
else:
    print "commandline"

__debug__
This constant is true if Python was not started with an -O option.

if __debug__:
    print 'Python started without optimization'