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

Noufal Ibrahim · Accepted Answer · 2012-08-03 16:12:51Z

5

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.

answered Aug 3, 2012 at 16:12

Noufal Ibrahim

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dfb · Accepted Answer · 2012-08-03 21:55:00Z

3

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

answered Aug 3, 2012 at 21:55

dfb

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Fábio Santos · Accepted Answer · 2013-01-31 11:03:58Z

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/

answered Jan 31, 2013 at 11:03

Fábio Santos

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2 Comments

dfb Over a year ago

Thats... sort of horrible, but interesting :)

Fábio Santos Over a year ago

@dfb Indeed. It also shows how to change the execution head to another line. Based upon that code, I implemented an absolute and relative JMP instruction. Quite fun.

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