Tracing the execution of a Python program line/statement by line programmatically

Question

What would be the easiest way to execute a Python script and, for each executed statement, pass the line number or the line itself to a callback function? For example

A piece of code to be executed

for i in range(5):
    z = i**2

and a callback

def callback(line):
   print line

The output would be:

for i in range(5):
    z = i**2
for i in range(5):
    z = i**2
for i in range(5):
    z = i**2
...
etc

Another way of stating this is that I want to know the piece of code that would be next to execute if I were running the script stepping through a debugger.. I've seen this question about tracing but I'm interested in doing more than tracing the number of times executed in the callback function.

A debugger is going to be helpful, but only if I can run it non-interactive mode with the ability to call back to the python code

Answer 1

Look at the sys.settrace function. This allows you to specify a tracing function which is executed for every line of code. It was, I think, specifically implemented to create pdb so it's at the right level of abstraction you seem to want. Implementing what you want will not be trivial but I think it's the best place to start.

For an production program that uses this, look at coverage.py.

Answer 2

Accepting Noufal's answer since it put me on the right path, but this helped as well

http://www.dalkescientific.com/writings/diary/archive/2005/04/20/tracing_python_code.html

Answer 3

The April's fools module goto is a working example of tracing implemented in Python, so although it's doubtlessly slower (than the C implementation in coverage.py), it's rather easy to understand. http://entrian.com/goto/