Trace Table For Python Programs

Is there a way to get the trace table for a Python program? Or for a program to run another program and get its trace table? I'm a teacher trying to flawlessly verify the answers t

Solution 1:

This isn't a use case that the current Python tracing tool support, but it should be possible to build. I don't know how you decide what columns to output. In your sample, a and b are the only local variables, but larger programs would have more interesting ecosystems of data.

Updated: here's a simple proof of concept:

 1     import sys
 2
 3     def trace(frame, event, arg_unused):
 4         print event, frame.f_lineno, frame.f_locals
 5         return trace
 6
 7     sys.settrace(trace)
 8
 9     def foo():
10         a = 1
11         b = 2
12
13         a = a + b
14
15     foo()

when run, the output is:

call 9 {}
line 10 {}
line 11 {'a': 1}
line 13 {'a': 1, 'b': 2}
return 13 {'a': 3, 'b': 2}

Solution 2:

You could use the Python debugger, though I do not know how to have it step through on it's own, but it's likely do-able, then you could just parse the output.

Here's a really crude example:

adding.py

a = 1b = 2a = a + b

running it...

PS >python -m pdb adding.py
> adding.py(1)<module>()
-> a = 1
(Pdb) alias stepprint step;;print a;;print b
(Pdb) stepprint
> adding.py(2)<module>()
-> b = 21
*** NameError: name 'b' is not defined
(Pdb) stepprint
> adding.py(4)<module>()
-> a = a + b
12
(Pdb) stepprint
--Return--
> adding.py(4)<module>()->None
-> a = a + b
32
(Pdb) stepprint
--Return--
> <string>(1)<module>()->None
32
(Pdb) stepprint
The program finished and will be restarted
> adding.py(1)<module>()
-> a = 1
*** NameError: name 'a' is not defined
*** NameError: name 'b' is not defined
(Pdb) q

PS >

End (q) on the "The program finished" bit.

Solution 3:

Based on what ned-batchelder proposed, as a teacher, I've made a Tracer class that help on creating LaTeX outputed longtable showing the trace of a program with selective variables, bypassing input() for automating process (especially while called by a \bash macro from the powerfull bashful LaTeX package).

tracer.py:

import sys
classTracer():
    def__init__(self, varList=[], startLine=1, jeuEssai=[]):
        """
        Arguments :
        \tvarList\ttraced variable list (used as column header)
        \tstartLine\toffset numbering line from the beginning of the program
        \tjeuEssai\tinput values to be sent to the automated input bypass
        """
        self.traced_variables = varList
        self.traced_line_start = startLine
        self.input_values = jeuEssai
        self.input_cursor = int(0)
        self.traced_variables_new_values = dict( (k, '') for k in self.traced_variables)

        print("\\begin{longtable}{c*{%i}{>{\\ttfamily}c}}" % len(self.traced_variables), file=sys.stderr, flush=True)
        print("\t\\hline\\no ligne",end='', file=sys.stderr)
        for header in self.traced_variables:
            print(" &", header,end='', file=sys.stderr)
        print(" \\\\ \\hline", file=sys.stderr)
        sys.settrace(self.tracer_programme_latex)


    deftracer_programme_latex(self, frame, event, args):
        if frame.f_code.co_name notin ['input','print','close']:
            if event == "line":
                output = str()
                for var in self.traced_variables:
                    current_val = str(frame.f_locals.get(var, "-"))
                    ifstr(self.traced_variables_new_values.get(var, "-")) != current_val:
                        self.traced_variables_new_values[var] = current_val
                        current_val = "\hit{}" + current_val
                    output += " & "
                    output += current_val
                output += " \\\\"print("\t%s%s" % (str(frame.f_lineno - self.traced_line_start), output), file=sys.stderr, flush=True)
        return self.tracer_programme_latex


    defclose(self):
        """Close the 'longtable' LaTeX environnement."""print("\\end{longtable}", file=sys.stderr, flush=True)


    definput(self, prompt=None):
        """
        bypass de la fonction 'input()' pour injecter
        les valeurs d'essais.
        Le jeu d'essai est fourni de manière cyclique. Cela peut
        causer des boucles infinies si vous ne fournissez pas une
        valeur permettant de réaliser l'arrêt des entrées (dans le
        cas bien-sûr où 'input()' est appelé dans une boucle).
        """
        self.input_cursor = (1 + self.input_cursor) % len(self.input_values)
        return self.input_values[self.input_cursor - 1]


    defprint(self, *args):
        pass

Next you can find an exemple,and the output generated:

program.py:

deffactor():
    question = "Give a number: "
    number = float(input(question))
    product = 1while number != 0 :
        product *= number
        print("Product:", product)
        number = float(input(question))

if __name__ == "__main__":
    import sys
    TRACING = len(sys.argv) == 2and sys.argv[1] == 'trace'if TRACING:
        from tracer import Tracer
        t = Tracer(varList=['question','number','product'], startLine=2, jeuEssai=[7,6,5,-8,0])
        input = t.input

    factor()
    if TRACING:
        t.close()

standard output: (while called by python3 program.py)

Give a number:7Product:7.0Give a number:6Product:42.0Give a number:5Product:210.0Give a number:-8Product:-1680.0Give a number:0

output with Tracer: (while called by python3 program.py trace 1>/dev/null)

\begin{longtable}{c*{3}{>{\ttfamily}c}}
    \hline\no ligne & question & number & product \\ \hline
    0 & \hit{}- & \hit{}- & \hit{}- \\
    1 & \hit{}Give a number:  & - & - \\
    2 & Give a number:  & \hit{}7.0 & - \\
    3 & Give a number:  & 7.0 & \hit{}1 \\
    4 & Give a number:  & 7.0 & 1 \\
    5 & Give a number:  & 7.0 & \hit{}7.0 \\
    6 & Give a number:  & 7.0 & 7.0 \\
    3 & Give a number:  & \hit{}6.0 & 7.0 \\
    4 & Give a number:  & 6.0 & 7.0 \\
    5 & Give a number:  & 6.0 & \hit{}42.0 \\
    6 & Give a number:  & 6.0 & 42.0 \\
    3 & Give a number:  & \hit{}5.0 & 42.0 \\
    4 & Give a number:  & 5.0 & 42.0 \\
    5 & Give a number:  & 5.0 & \hit{}210.0 \\
    6 & Give a number:  & 5.0 & 210.0 \\
    3 & Give a number:  & \hit{}-8.0 & 210.0 \\
    4 & Give a number:  & -8.0 & 210.0 \\
    5 & Give a number:  & -8.0 & \hit{}-1680.0 \\
    6 & Give a number:  & -8.0 & -1680.0 \\
    3 & Give a number:  & \hit{}0.0 & -1680.0 \\
\end{longtable}

The \hit{} macro is insert while the value has changed. It's up to you to define something relevant, like a coloring macro : \newcommand{\hit}{\color{red}}