Trace or track Python statement execution (trace)

Function in the 'trace' module in Python library generates trace of program execution, and annotated statement coverage. It also has functions to list functions called during run by generating caller relationships.

Following two Python scripts are used as an example to demonstrate features of trace module.

#myfunctions.py
import math
def area(x):
   a = math.pi*math.pow(x,2)
   return a
def factorial(x):
   if x==1:
      return 1
   else:
return x*factorial(x-1)

#mymain.py
import myfunctions
def main():
   x = 5
   print ('area=',myfunctions.area(x))
   print ('factorial=',myfunctions.factorial(x))

if __name__=='__main__':
   main()

The 'trace' module has a command line interface. All the functions in the module can be called using command line switches. The most important option is --trace which displays program lines as they are executed. In following example another option --ignore-dir is used. It ignores specified directories while generating the trace.

E:\python37>python -m trace --ignore-dir=../lib --trace mymain.py

Output

mymain.py(2): def main():
mymain.py(7): if __name__=='__main__':
mymain.py(8): main()
--- modulename: mymain, funcname: main
mymain.py(3): x=5
mymain.py(4): print ('area=',myfunctions.area(x))
--- modulename: myfunctions, funcname: area
myfunctions.py(3): a=math.pi*math.pow(x,2)
myfunctions.py(4): return a
area= 78.53981633974483
mymain.py(5): print ('factorial=',myfunctions.factorial(x))
--- modulename: myfunctions, funcname: factorial
myfunctions.py(6): if x==1:
myfunctions.py(9): return x*factorial(x-1)
--- modulename: myfunctions, funcname: factorial
myfunctions.py(6): if x==1:
myfunctions.py(9): return x*factorial(x-1)
--- modulename: myfunctions, funcname: factorial
myfunctions.py(6): if x==1:
myfunctions.py(9): return x*factorial(x-1)
--- modulename: myfunctions, funcname: factorial
myfunctions.py(6): if x==1:
myfunctions.py(9): return x*factorial(x-1)
--- modulename: myfunctions, funcname: factorial
myfunctions.py(6): if x==1:
myfunctions.py(7): return 1
factorial= 120

The --count option generates a file for each module in use with, cover extension.

E:\python37>python -m trace --count mymain.py
area= 78.53981633974483
factorial = 120

myfunctions.cover

1: import math
1: def area(x):
1:    a = math.pi*math.pow(x,2)
1:    return a
1: def factorial(x):
5:    if x==1:
1:       return 1
   else:
4:    return x*factorial(x-1)

mymain.cover

1: import myfunctions
1: def main():
1:    x = 5
1:    print ('area=',myfunctions.area(x))
1:    print ('factorial=',myfunctions.factorial(x))

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

--summary option displays a brief summary if –count option is also used.

E:\python37>python -m trace --count --summary mymain.py
area = 78.53981633974483
factorial = 120
lines cov% module (path)
   8 100% myfunctions (E:\python37\myfunctions.py)
   7 100% mymain (mymain.py)

The --file option specifies name of file in which accumulates count over several tracing runs.

E:\python37>python -m trace --count --file report.txt mymain.py
area = 78.53981633974483
factorial = 120
Skipping counts file 'report.txt': [Errno 2] No such file or directory: 'report.txt'

E:\python37>python -m trace --count --file report.txt mymain.py
area= 78.53981633974483
factorial= 120

--listfuncs option displays functions called during execution of program.

E:\python37>python -m trace --listfunc mymain.py | findstr -v importlib
area= 78.53981633974483
factorial= 120

functions called:
filename: E:\python37\lib\encodings\cp1252.py, modulename: cp1252, funcname: IncrementalEncoder.encode
filename: E:\python37\myfunctions.py, modulename: myfunctions, funcname: <module>
filename: E:\python37\myfunctions.py, modulename: myfunctions, funcname: area
filename: E:\python37\myfunctions.py, modulename: myfunctions, funcname: factorial
filename: mymain.py, modulename: mymain, funcname: <module>
filename: mymain.py, modulename: mymain, funcname: main

--trackcalls option is used along with –list funcs option. It generates calling relationships.

E:\python37>python -m trace --listfunc --trackcalls mymain.py | findstr -v importlib
area= 78.53981633974483
factorial= 120

calling relationships:

--> E:\python37\myfunctions.py


*** E:\python37\lib\trace.py ***
--> mymain.py
trace.Trace.runctx -> mymain.<module>

*** E:\python37\myfunctions.py ***
myfunctions.factorial -> myfunctions.factorial

*** mymain.py ***
mymain.<module> -> mymain.main
--> E:\python37\lib\encodings\cp1252.py
mymain.main -> cp1252.IncrementalEncoder.encode
--> E:\python37\myfunctions.py
mymain.main -> myfunctions.area
mymain.main -> myfunctions.factorial

(ref: https://www.tutorialspoint.com/trace-or-track-python-statement-execution-trace)