pytrace is a simple program to print out a trace of a Python program as it runs. That is, it displays the values of variables as they are modified (although it does not display all variables by default).
pytrace is currently distributed as a single file – download it and put it somewhere you can execute it easily. pytrace was written and tested with Python 3.6; higher versions may work, but lower versions will not.
Simply run pytrace.py with the name of the file you'd like to trace.
For example:
python3.6 pytrace.py my_script.py
This would produce output that looks something like this:
--> <module>
<no variables>
a = 1
a = 1, b = 2
a = 3, b = 2
<-- <module> (returned None)
More options are available; see the output of pytrace.py -h.
pytrace was written in an afternoon, and is intended as a demonstration of Python's debugging capabilities. It is not meant as a serious debugging tool. As such, there are several situations where it is known to produce incorrect output, and it's likely that there are many more ways to confuse pytrace into misbehaving.
If your code uses the global keyword, pytrace is likely to display the
wrong value of variables at various points. Any modifications to global
variables won't be shown until the end of the scope in which they're
made global. A concrete example:
foo = "global value"
bar = "global value"
def do_something():
global foo
foo = "local value"
bar = "local value"
do_something()The above code is traced like this:
--> <module>
<no variables>
foo = 'global value'
foo = 'global value', bar = 'global value'
--> do_something
bar = 'local value', foo = 'global value'
<-- do_something (returned None)
foo = 'local value', bar = 'global value'
<-- <module> (returned None)
Note the missing change from foo = "global value" to foo = "local value" inside do_something, and the fact that it is displayed
correctly after the function returns.
Further, pytrace has not been tested at all with imports or classes; it's likely that, due to pytrace's simple idea of scope, some variables will be displayed incorrectly.
Patches to improve this are welcome!
Copyright © 2018 Ash Holland. Licensed under the EUPL (1.2 or later).
