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24. The Python Debugger

24. The Python Debugger

The module pdb defines an interactive source code debugger for Python programs. It supports setting (conditional) breakpoints and single stepping at the source line level, inspection of stack frames, source code listing, and evaluation of arbitrary Python code in the context of any stack frame. It also supports post-mortem debugging and can be called under program control.

The debugger is extensible -- it is actually defined as the class Pdb. This is currently undocumented but easily understood by reading the source. The extension interface uses the modules bdb (undocumented) and cmd.

The debugger's prompt is "(Pdb) ". Typical usage to run a program under control of the debugger is:

>>> import pdb
>>> import mymodule
>>> pdb.run('mymodule.test()')
> <string>(0)?()
(Pdb) continue
> <string>(1)?()
(Pdb) continue
NameError: 'spam'
> <string>(1)?()

pdb.py can also be invoked as a script to debug other scripts. For example:

python -m pdb myscript.py

When invoked as a script, pdb will automatically enter post-mortem debugging if the program being debugged exits abnormally. After post-mortem debugging (or after normal exit of the program), pdb will restart the program. Automatic restarting preserves pdb's state (such as breakpoints) and in most cases is more useful than quitting the debugger upon program's exit. New in version 2.4: Restarting post-mortem behavior added.

Typical usage to inspect a crashed program is:

>>> import pdb
>>> import mymodule
>>> mymodule.test()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "./mymodule.py", line 4, in test
  File "./mymodule.py", line 3, in test2
    print spam
NameError: spam
>>> pdb.pm()
> ./mymodule.py(3)test2()
-> print spam

The module defines the following functions; each enters the debugger in a slightly different way:

run( statement[, globals[, locals]])
Execute the statement (given as a string) under debugger control. The debugger prompt appears before any code is executed; you can set breakpoints and type "continue", or you can step through the statement using "step" or "next" (all these commands are explained below). The optional globals and locals arguments specify the environment in which the code is executed; by default the dictionary of the module __main__ is used. (See the explanation of the exec statement or the eval() built-in function.)

runeval( expression[, globals[, locals]])
Evaluate the expression (given as a string) under debugger control. When runeval() returns, it returns the value of the expression. Otherwise this function is similar to run().

runcall( function[, argument, ...])
Call the function (a function or method object, not a string) with the given arguments. When runcall() returns, it returns whatever the function call returned. The debugger prompt appears as soon as the function is entered.

set_trace( )
Enter the debugger at the calling stack frame. This is useful to hard-code a breakpoint at a given point in a program, even if the code is not otherwise being debugged (e.g. when an assertion fails).

post_mortem( traceback)
Enter post-mortem debugging of the given traceback object.

pm( )
Enter post-mortem debugging of the traceback found in sys.last_traceback.

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