instrument.py

import sys


source_code = \
"""
def print_message(s):
	print(s)

def add_numbers(x,y):
	return x + y

def main():
	print_message('hello world')
	sum = add_numbers(3,4)
	print_message('sum of numbers = %s' % str(sum))

if __name__=='__main__':
	main()
"""


def instrument(file_name, s):
    source_lines = s.splitlines()
    instrumented_lines = 'import instrumentation'
    instrumented_lines += '\n'
    instrumented_lines += '\n'
    instrument_line = False
    fn_name = ''

    for source_line in source_lines:
    	stripped_source_line = source_line.strip()

    	if len(stripped_source_line) > 0 and stripped_source_line[0] == '#':
    		stripped_source_line = ''

    	if len(stripped_source_line) == 0:
    		instrumented_lines += source_line
    		instrumented_lines += '\n'
    	else:
    		if instrument_line:
    			num_whitespace_chars = len(source_line) - len(stripped_source_line)
    			whitespace = source_line[0:num_whitespace_chars]
    			injected_line = whitespace
    			injected_line += "instrumentation.trace_fn('%s','%s','%s')" % (file_name, '', fn_name)
    			instrumented_lines += injected_line
    			instrumented_lines += '\n'
    			instrument_line = False
    			fn_name = ''
    		pos_def = stripped_source_line.find('def ')
    		if pos_def == 0:
    			instrumented_lines += source_line
    			instrumented_lines += '\n'
    			pos_paren = stripped_source_line.find('(')
    			if pos_paren > 0:
    				fn_name = stripped_source_line[4:pos_paren]
    				instrument_line = True
    			else:
    				print('error: unable to parse python source')
    				sys.exit(1)
    		else:
    			instrumented_lines += source_line
    			instrumented_lines += '\n'

    return instrumented_lines

def main():
	instrumented_code = instrument('example.py', source_code)
	print(instrumented_code)

if __name__=='__main__':
	main()

instrumentation.py

class Trace:
	def __init__(self, file_name, class_name, method_name):
		self.file_name = file_name
		self.class_name = class_name
		self.method_name = method_name

	def to_string(self):
		s = 'file=%s, class=%s, method=%s' % (self.file_name, self.class_name, self.method_name)
		return s


def file_trace_fn(trace):
	pass


def print_trace_fn(trace):
	print(trace.to_string())


def trace_fn(file_name, class_name, method_name):
	trace = Trace(file_name, class_name, method_name)
	print_trace_fn(trace)
	file_trace_fn(trace)

python instrument.py > instrumented.py

python instrumented.py
trace_fn: file=example.py, class=, method=main
trace_fn: file=example.py, class=, method=print_message
hello world
trace_fn: file=example.py, class=, method=add_numbers
trace_fn: file=example.py, class=, method=print_message
sum of numbers = 7