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pdb – Interactive Debugger

Purpose:Python’s Interactive Debugger
Available In:1.4 and later

pdb implements an interactive debugging environment for Python programs. It includes features to let you pause your program, look at the values of variables, and watch program execution step-by-step, so you can understand what your program actually does and find bugs in the logic.

Starting the Debugger

The first step to using pdb is causing the interpreter to enter the debugger when you want it to. There are a few different ways to do that, depending on your starting conditions and what you need to debug.

From the Command Line

The most straightforward way to use the debugger is to run it from the command line, giving it your own program as input so it knows what to run.

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#!/usr/bin/env python
# encoding: utf-8
#
# Copyright (c) 2010 Doug Hellmann.  All rights reserved.
#

class MyObj(object):

    def __init__(self, num_loops):
        self.count = num_loops

    def go(self):
        for i in range(self.count):
            print i
        return

if __name__ == '__main__':
    MyObj(5).go()

Running the debugger from the command line causes it to load your source file and stop execution on the first statement it finds. In this case, it stops before evaluating the definition of the class MyObj on line 7.

$ python -m pdb pdb_script.py
> .../pdb_script.py(7)<module>()
-> """
(Pdb)

Note

Normally pdb includes the full path to each module in the output when printing a filename. In order to maintain clear examples, the sample output in this section replaces the path with ....

Within the Interpreter

Many Python developers work with the interactive interpreter while developing early versions of modules because it lets them experiment more iteratively without the save/run/repeat cycle needed when creating standalone scripts. To run the debugger from within an interactive interpreter, use run() or runeval().

$ python
Python 2.7 (r27:82508, Jul  3 2010, 21:12:11)
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import pdb_script
>>> import pdb
>>> pdb.run('pdb_script.MyObj(5).go()')
> <string>(1)<module>()
(Pdb)

The argument to run() is a string expression that can be evaluated by the Python interpreter. The debugger will parse it, then pause execution just before the first expression evaluates. You can use the debugger commands described below to navigate and control the execution.

From Within Your Program

Both of the previous examples assume you want to start the debugger at the beginning of your program. For a long-running process where the problem appears much later in the program execution, it will be more convenient to start the debugger from inside your program using set_trace().

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#!/usr/bin/env python
# encoding: utf-8
#
# Copyright (c) 2010 Doug Hellmann.  All rights reserved.
#

import pdb

class MyObj(object):

    def __init__(self, num_loops):
        self.count = num_loops

    def go(self):
        for i in range(self.count):
            pdb.set_trace()
            print i
        return

if __name__ == '__main__':
    MyObj(5).go()

Line 16 of the sample script triggers the debugger at that point in execution.

$ python ./pdb_set_trace.py
> .../pdb_set_trace.py(17)go()
-> print i
(Pdb)

set_trace() is just a Python function, so you can call it at any point in your program. This lets you enter the debugger based on conditions inside your program, including from an exception handler or via a specific branch of a control statement.

After a Failure

Debugging a failure after a program terminates is called post-mortem debugging. pdb supports post-mortem debugging through the pm() and post_mortem() functions.

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#!/usr/bin/env python
# encoding: utf-8
#
# Copyright (c) 2010 Doug Hellmann.  All rights reserved.
#

class MyObj(object):

    def __init__(self, num_loops):
        self.count = num_loops

    def go(self):
        for i in range(self.num_loops):
            print i
        return

Here the incorrect attribute name on line 13 triggers an AttributeError exception, causing execution to stop. pm() looks for the active traceback and starts the debugger at the point in the call stack where the exception occurred.

$ python
Python 2.7 (r27:82508, Jul  3 2010, 21:12:11)
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from pdb_post_mortem import MyObj
>>> MyObj(5).go()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pdb_post_mortem.py", line 13, in go
    for i in range(self.num_loops):
AttributeError: 'MyObj' object has no attribute 'num_loops'
>>> import pdb
>>> pdb.pm()
> .../pdb_post_mortem.py(13)go()
-> for i in range(self.num_loops):
(Pdb)

Controlling the Debugger

You interact with the debugger using a small command language that lets you move around the call stack, examine and change the values of variables, and control how the debugger executes your program. The interactive debugger uses readline to accept commands. Entering a blank line re-runs the previous command again, unless it was a list operation.

Examining Variables on the Stack

Each frame on the stack maintains a set of variables, including values local to the function being executed and global state information. pdb provides several ways to examine the contents of those variables.

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#!/usr/bin/env python
# encoding: utf-8
#
# Copyright (c) 2010 Doug Hellmann.  All rights reserved.
#

import pdb

def recursive_function(n=5, output='to be printed'):
    if n > 0:
        recursive_function(n-1)
    else:
        pdb.set_trace()
        print output
    return

if __name__ == '__main__':
    recursive_function()

The args command (abbreviated a) prints all of the arguments to the function active in the current frame. This example also uses a recursive function to show what a deeper stack looks like when printed by where.

$ python pdb_function_arguments.py
> .../pdb_function_arguments.py(14)recursive_function()
-> return
(Pdb) where
  .../pdb_function_arguments.py(17)<module>()
-> recursive_function()
  .../pdb_function_arguments.py(11)recursive_function()
-> recursive_function(n-1)
  .../pdb_function_arguments.py(11)recursive_function()
-> recursive_function(n-1)
  .../pdb_function_arguments.py(11)recursive_function()
-> recursive_function(n-1)
  .../pdb_function_arguments.py(11)recursive_function()
-> recursive_function(n-1)
  .../pdb_function_arguments.py(11)recursive_function()
-> recursive_function(n-1)
> .../pdb_function_arguments.py(14)recursive_function()
-> return

(Pdb) args
n = 0
output = to be printed

(Pdb) up
> .../pdb_function_arguments.py(11)recursive_function()
-> recursive_function(n-1)

(Pdb) args
n = 1
output = to be printed

(Pdb)

The p command evaluates an expression given as argument and prints the result. You can also use Python’s print statement, but that is passed through to the interpreter to be executed rather than running as a command in the debugger.

(Pdb) p n
1

(Pdb) print n
1

Similarly, prefixing an expression with ! passes it to the Python interpreter to be evaluated. You can use this feature to execute arbitrary Python statements, including modifying variables. This example changes the value of output before letting the debugger continue running the program. The next statement after the call to set_trace() prints the value of output, showing the modified value.

$ python pdb_function_arguments.py
> .../pdb_function_arguments.py(14)recursive_function()
-> print output

(Pdb) !output
'to be printed'

(Pdb) !output='changed value'

(Pdb) continue
changed value

For more complicated values such as nested or large data structures, use pp to “pretty print” them. This program reads several lines of text from a file.

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#!/usr/bin/env python
# encoding: utf-8
#
# Copyright (c) 2010 Doug Hellmann.  All rights reserved.
#

import pdb

with open('lorem.txt', 'rt') as f:
    lines = f.readlines()

pdb.set_trace()

Printing the variable lines with p results in output that is difficult to read because it wraps awkwardly. pp uses pprint to format the value for clean printing.

$ python pdb_pp.py
--Return--
> .../pdb_pp.py(12)<module>()->None
-> pdb.set_trace()
(Pdb) p lines
['Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Donec\n', 'egestas, enim
et consectetuer ullamcorper, lectus ligula rutrum leo, a\n', 'elementum elit tortor
eu quam.\n']

(Pdb) pp lines
['Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Donec\n',
 'egestas, enim et consectetuer ullamcorper, lectus ligula rutrum leo, a\n',
 'elementum elit tortor eu quam.\n']

(Pdb)

Stepping Through Your Program

In addition to navigating up and down the call stack when the program is paused, you can also step through execution of the program past the point where it enters the debugger.

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#!/usr/bin/env python
# encoding: utf-8
#
# Copyright (c) 2010 Doug Hellmann.  All rights reserved.
#

import pdb

def f(n):
    for i in range(n):
        j = i * n
        print i, j
    return

if __name__ == '__main__':
    pdb.set_trace()
    f(5)

Use step to execute the current line and then stop at the next execution point – either the first statement inside a function being called or the next line of the current function.

$ python pdb_step.py
> /Users/dhellmann/Documents/PyMOTW/src.pdb/PyMOTW/pdb/pdb_step.py(17)<module>()
-> f(5)

The interpreter pauses at the call to set_trace() and gives control to the debugger. The first step causes the execution to enter f().

(Pdb) step
--Call--
> .../pdb_step.py(9)f()
-> def f(n):

One more step moves execution to the first line of f() and starts the loop.

(Pdb) step
> /Users/dhellmann/Documents/PyMOTW/src.pdb/PyMOTW/pdb/pdb_step.py(10)f()
-> for i in range(n):

Stepping again moves to the first line inside the loop where j is defined.

(Pdb) step
> /Users/dhellmann/Documents/PyMOTW/src.pdb/PyMOTW/pdb/pdb_step.py(11)f()
-> j = i * n
(Pdb) p i
0

The value of i is 0, so after one more step the value of j should also be 0.

(Pdb) step
> /Users/dhellmann/Documents/PyMOTW/src.pdb/PyMOTW/pdb/pdb_step.py(12)f()
-> print i, j

(Pdb) p j
0

(Pdb)

Stepping one line at a time like this can become tedious if there is a lot of code to cover before the point where the error occurs, or if the same function is called repeatedly.

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#!/usr/bin/env python
# encoding: utf-8
#
# Copyright (c) 2010 Doug Hellmann.  All rights reserved.
#

import pdb

def calc(i, n):
    j = i * n
    return j

def f(n):
    for i in range(n):
        j = calc(i, n)
        print i, j
    return

if __name__ == '__main__':
    pdb.set_trace()
    f(5)

In this example, there is nothing wrong with calc(), so stepping through it each time it is called in the loop in f() obscures the useful output by showing all of the lines of calc() as they are executed.

$ python pdb_next.py
> .../pdb_next.py(21)<module>()
-> f(5)
(Pdb) step
--Call--
> .../pdb_next.py(13)f()
-> def f(n):

(Pdb) step
> .../pdb_next.py(14)f()
-> for i in range(n):

(Pdb) step
> .../pdb_next.py(15)f()
-> j = calc(i, n)

(Pdb) step
--Call--
> .../pdb_next.py(9)calc()
-> def calc(i, n):

(Pdb) step
> .../pdb_next.py(10)calc()
-> j = i * n

(Pdb) step
> .../pdb_next.py(11)calc()
-> return j

(Pdb) step
--Return--
> .../pdb_next.py(11)calc()->0
-> return j

(Pdb) step
> .../pdb_next.py(16)f()
-> print i, j

(Pdb) step
0 0

The next command is like step, but does not enter functions called from the statement being executed. In effect, it steps all the way through the function call to the next statement in the current function in a single operation.

> .../pdb_next.py(14)f()
-> for i in range(n):
(Pdb) step
> .../pdb_next.py(15)f()
-> j = calc(i, n)

(Pdb) next
> .../pdb_next.py(16)f()
-> print i, j

(Pdb)

The until command is like next, except it explicitly continues until execution reaches a line in the same function with a line number higher than the current value. That means, for example, that until can be used to step past the end of a loop.

$ python pdb_next.py
> .../pdb_next.py(21)<module>()
-> f(5)
(Pdb) step
--Call--
> .../pdb_next.py(13)f()
-> def f(n):

(Pdb) step
> .../pdb_next.py(14)f()
-> for i in range(n):

(Pdb) step
> .../pdb_next.py(15)f()
-> j = calc(i, n)

(Pdb) next
> .../pdb_next.py(16)f()
-> print i, j

(Pdb) until
0 0
1 5
2 10
3 15
4 20
> .../pdb_next.py(17)f()
-> return

(Pdb)

Before until was run, the current line was 16, the last line of the loop. After until ran, execution was on line 17, and the loop had been exhausted.

return is another short-cut for bypassing parts of a function. It continues executing until the function is about to execute a return statement, and then it pauses. This gives you time to look at the return value before the function returns.

$ python pdb_next.py
> .../pdb_next.py(21)<module>()
-> f(5)
(Pdb) step
--Call--
> .../pdb_next.py(13)f()
-> def f(n):

(Pdb) step
> .../pdb_next.py(14)f()
-> for i in range(n):

(Pdb) return
0 0
1 5
2 10
3 15
4 20
--Return--
> .../pdb_next.py(17)f()->None
-> return

(Pdb)

Breakpoints

As programs grow even longer, even using next and until will become slow and cumbersome. Instead of stepping through the program by hand, a better solution is to let it run normally until it reaches a point where you want the debugger to interrupt it. You could use set_trace() to start the debugger, but that only works if there is only one point you want to pause the program. It is more convenient to run the program through the debugger, but tell the debugger where to stop in advance using breakpoints. The debugger monitors your program, and when it reaches the location described by a breakpoint the program is paused before the line is executed.

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#!/usr/bin/env python
# encoding: utf-8
#
# Copyright (c) 2010 Doug Hellmann.  All rights reserved.
#

def calc(i, n):
    j = i * n
    print 'j =', j
    if j > 0:
        print 'Positive!'
    return j

def f(n):
    for i in range(n):
        print 'i =', i
        j = calc(i, n)
    return

if __name__ == '__main__':
    f(5)

There are several options to the break command used for setting break points. You can specify the line number, file, and function where processing should pause. To set a breakpoint on a specific line of the current file, use break lineno:

$ python -m pdb pdb_break.py
> .../pdb_break.py(7)<module>()
-> def calc(i, n):
(Pdb) break 11
Breakpoint 1 at .../pdb_break.py:11

(Pdb) continue
i = 0
j = 0
i = 1
j = 5
> .../pdb_break.py(11)calc()
-> print 'Positive!'

(Pdb)

The command continue tells the debugger to keep running your program until the next breakpoint. In this case, it runs through the first iteration of the for loop in f() and stops inside calc() during the second iteration.

Breakpoints can also be set to the first line of a function by specifying the function name instead of a line number. This example shows what happens if a breakpoint is added for the calc() function.

$ python -m pdb pdb_break.py
> .../pdb_break.py(7)<module>()
-> def calc(i, n):
(Pdb) break calc
Breakpoint 1 at .../pdb_break.py:7

(Pdb) continue
i = 0
> .../pdb_break.py(8)calc()
-> j = i * n

(Pdb) where
  .../pdb_break.py(21)<module>()
-> f(5)
  .../pdb_break.py(17)f()
-> j = calc(i, n)
> .../pdb_break.py(8)calc()
-> j = i * n

(Pdb)

To specify a breakpoint in another file, prefix the line or function argument with a filename.

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#!/usr/bin/env python
# encoding: utf-8
#
# Copyright (c) 2010 Doug Hellmann.  All rights reserved.
#

from pdb_break import f

f(5)

Here a breakpoint is set for line 11 of pdb_break.py after starting the main program pdb_break_remote.py.

$ python -m pdb pdb_break_remote.py
> .../pdb_break_remote.py(7)<module>()
-> from pdb_break import f
(Pdb) break pdb_break.py:11
Breakpoint 1 at .../pdb_break.py:11

(Pdb) continue
i = 0
j = 0
i = 1
j = 5
> .../pdb_break.py(11)calc()
-> print 'Positive!'

(Pdb)

The filename can be a full path to the source file, or a relative path to a file available on sys.path.

To list the breakpoints currently set, use break without any arguments. The output includes the file and line number of each break point, as well as information about how many times it has been encountered.

$ python -m pdb pdb_break.py
> .../pdb_break.py(7)<module>()
-> def calc(i, n):
(Pdb) break 11
Breakpoint 1 at .../pdb_break.py:11

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_break.py:11

(Pdb) continue
i = 0
j = 0
i = 1
j = 5
> .../pdb/pdb_break.py(11)calc()
-> print 'Positive!'

(Pdb) continue
Positive!
i = 2
j = 10
> .../pdb_break.py(11)calc()
-> print 'Positive!'

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_break.py:11
        breakpoint already hit 2 times

(Pdb)

Managing Breakpoints

As each new breakpoint is added, it is assigned a numerical identifier. These ID numbers are used to enable, disable, and remove the breakpoints interactively.

Turning off a breakpoint with disable tells the debugger not to stop when that line is reached. The breakpoint is remembered, but ignored.

$ python -m pdb pdb_break.py
> .../pdb_break.py(7)<module>()
-> def calc(i, n):
(Pdb) break calc
Breakpoint 1 at .../pdb_break.py:7

(Pdb) break 11
Breakpoint 2 at .../pdb_break.py:11

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_break.py:7
2   breakpoint   keep yes   at .../pdb_break.py:11

(Pdb) disable 1

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep no    at .../pdb_break.py:7
2   breakpoint   keep yes   at .../pdb_break.py:11

(Pdb) continue
i = 0
j = 0
i = 1
j = 5
> .../pdb_break.py(11)calc()
-> print 'Positive!'

(Pdb)

The debugging session below sets two breakpoints in the program, then disables one. The program is run until the remaining breakpoint is encountered, and then the other breakpoint is turned back on with enable before execution continues.

$ python -m pdb pdb_break.py
> .../pdb_break.py(7)<module>()
-> def calc(i, n):
(Pdb) break calc
Breakpoint 1 at .../pdb_break.py:7

(Pdb) break 16
Breakpoint 2 at .../pdb_break.py:16

(Pdb) disable 1

(Pdb) continue
> .../pdb_break.py(16)f()
-> print 'i =', i

(Pdb) list
 11             print 'Positive!'
 12         return j
 13
 14     def f(n):
 15         for i in range(n):
 16 B->         print 'i =', i
 17             j = calc(i, n)
 18         return
 19
 20     if __name__ == '__main__':
 21         f(5)

(Pdb) continue
i = 0
j = 0
> .../pdb_break.py(16)f()
-> print 'i =', i

(Pdb) list
 11             print 'Positive!'
 12         return j
 13
 14     def f(n):
 15         for i in range(n):
 16 B->         print 'i =', i
 17             j = calc(i, n)
 18         return
 19
 20     if __name__ == '__main__':
 21         f(5)

(Pdb) p i
1

(Pdb) enable 1

(Pdb) continue
i = 1
> .../pdb_break.py(8)calc()
-> j = i * n

(Pdb) list
  3     #
  4     # Copyright (c) 2010 Doug Hellmann.  All rights reserved.
  5     #
  6
  7 B   def calc(i, n):
  8  ->     j = i * n
  9         print 'j =', j
 10         if j > 0:
 11             print 'Positive!'
 12         return j
 13

(Pdb)

The lines prefixed with B in the output from list show where the breakpoints are set in the program (lines 9 and 18).

Use clear to delete a breakpoint entirely.

$ python -m pdb pdb_break.py
> .../pdb_break.py(7)<module>()
-> def calc(i, n):
(Pdb) break calc
Breakpoint 1 at .../pdb_break.py:7

(Pdb) break 11
Breakpoint 2 at .../pdb_break.py:11

(Pdb) break 16
Breakpoint 3 at .../pdb_break.py:16

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_break.py:7
2   breakpoint   keep yes   at .../pdb_break.py:11
3   breakpoint   keep yes   at .../pdb_break.py:16

(Pdb) clear 2
Deleted breakpoint 2

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_break.py:7
3   breakpoint   keep yes   at .../pdb_break.py:16

(Pdb)

The other breakpoints retain their original identifiers and are not renumbered.

Temporary Breakpoints

A temporary breakpoint is automatically cleared the first time program execution hits it. Using a temporary breakpoint lets you reach a particular spot in the program flow quickly, just as with a regular breakpoint, but since it is cleared immediately it does not interfere with subsequent progress if that part of the program is run repeatedly.

$ python -m pdb pdb_break.py
> .../pdb_break.py(7)<module>()
-> def calc(i, n):
(Pdb) tbreak 11
Breakpoint 1 at .../pdb_break.py:11

(Pdb) continue
i = 0
j = 0
i = 1
j = 5
Deleted breakpoint 1
> .../pdb_break.py(11)calc()
-> print 'Positive!'

(Pdb) break

(Pdb) continue
Positive!
i = 2
j = 10
Positive!
i = 3
j = 15
Positive!
i = 4
j = 20
Positive!
The program finished and will be restarted
> .../pdb_break.py(7)<module>()
-> def calc(i, n):

(Pdb)

After the program reaches line 11 the first time, the breakpoint is removed and execution does not stop again until the program finishes.

Conditional Breakpoints

Rules can be applied to breakpoints so that execution only stops when the conditions are met. Using conditional breakpoints gives you finer control over how the debugger pauses your program than manually enabling and disabling breakpoints yourself.

Conditional breakpoints can be set in two ways. The first is to specify the condition when the breakpoint is set using break.

$ python -m pdb pdb_break.py
> .../pdb_break.py(7)<module>()
-> def calc(i, n):
(Pdb) break 9, j>0
Breakpoint 1 at .../pdb_break.py:9

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_break.py:9
        stop only if j>0

(Pdb) continue
i = 0
j = 0
i = 1
> .../pdb_break.py(9)calc()
-> print 'j =', j

(Pdb)

The condition argument must be an expression using values visible in the stack frame where the breakpoint is defined. If the expression evaluates as true, execution stops at the breakpoint.

A condition can also be applied to an existing breakpoint using the condition command. The arguments are the breakpoint ID and the expression.

$ python -m pdb pdb_break.py
> .../pdb_break.py(7)<module>()
-> def calc(i, n):
(Pdb) break 9
Breakpoint 1 at .../pdb_break.py:9

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_break.py:9

(Pdb) condition 1 j>0

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_break.py:9
        stop only if j>0

(Pdb)

Ignoring Breakpoints

Programs with a lot of looping or recursive calls to the same function are often easier to debug by “skipping ahead” in the execution, instead of watching every call or breakpoint. The ignore command tells the debugger to pass over a breakpoint without stopping. Each time processing encounteres the breakpoint, it decrements the ignore counter. When the counter is zero, the breakpoint is re-activated.

$ python -m pdb pdb_break.py
> .../pdb_break.py(7)<module>()
-> def calc(i, n):
(Pdb) break 17
Breakpoint 1 at .../pdb_break.py:17

(Pdb) continue
i = 0
> .../pdb_break.py(17)f()
-> j = calc(i, n)

(Pdb) next
j = 0
> .../pdb_break.py(15)f()
-> for i in range(n):

(Pdb) ignore 1 2
Will ignore next 2 crossings of breakpoint 1.

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_break.py:17
        ignore next 2 hits
        breakpoint already hit 1 time

(Pdb) continue
i = 1
j = 5
Positive!
i = 2
j = 10
Positive!
i = 3
> .../pdb_break.py(17)f()
-> j = calc(i, n)

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_break.py:17
        breakpoint already hit 4 times

Explicitly resetting the ignore count to zero re-enables the breakpoint immediately.

$ python -m pdb pdb_break.py
> .../pdb_break.py(7)<module>()
-> def calc(i, n):
(Pdb) break 17
Breakpoint 1 at .../pdb_break.py:17

(Pdb) ignore 1 2
Will ignore next 2 crossings of breakpoint 1.

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_break.py:17
        ignore next 2 hits

(Pdb) ignore 1 0
Will stop next time breakpoint 1 is reached.

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_break.py:17

Triggering Actions on a Breakpoint

In addition to the purely interactive mode, pdb supports basic scripting. Using commands, you can define a series of interpreter commands, including Python statements, to be executed when a specific breakpoint is encountered. After running commands with the breakpoint number as argument, the debugger prompt changes to (com). Enter commands one a time, and finish the list with end to save the script and return to the main debugger prompt.

$ python -m pdb pdb_break.py
> .../pdb_break.py(7)<module>()
-> def calc(i, n):
(Pdb) break 9
Breakpoint 1 at .../pdb_break.py:9

(Pdb) commands 1
(com) print 'debug i =', i
(com) print 'debug j =', j
(com) print 'debug n =', n
(com) end

(Pdb) continue
i = 0
debug i = 0
debug j = 0
debug n = 5
> .../pdb_break.py(9)calc()
-> print 'j =', j

(Pdb) continue
j = 0
i = 1
debug i = 1
debug j = 5
debug n = 5
> .../pdb_break.py(9)calc()
-> print 'j =', j

(Pdb)

This feature is especially useful for debugging code that uses a lot of data structures or variables, since you can have the debugger print out all of the values automatically, instead of doing it manually each time the breakpoint is encountered.

Changing Execution Flow

The jump command lets you alter the flow of your program at runtime, without modifying the code. You can skip forwards to avoid running some code, or backwards to run it again. This sample program generates a list of numbers.

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#!/usr/bin/env python
# encoding: utf-8
#
# Copyright (c) 2010 Doug Hellmann.  All rights reserved.
#

def f(n):
    result = []
    j = 0
    for i in range(n):
        j = i * n + j
        j += n
        result.append(j)
    return result

if __name__ == '__main__':
    print f(5)

When run without interference the output is a sequence of increasing numbers divisible by 5.

$ python pdb_jump.py
[5, 15, 30, 50, 75]

Jump Ahead

Jumping ahead moves the point of execution past the current location without evaluating any of the statements in between. By skipping over line 13 in the example below, the value of j is not incremented and all of the subsequent values that depend on it are a little smaller.

$ python -m pdb pdb_jump.py
> .../pdb_jump.py(7)<module>()
-> def f(n):
(Pdb) break 12
Breakpoint 1 at .../pdb_jump.py:12

(Pdb) continue
> .../pdb_jump.py(12)f()
-> j += n

(Pdb) p j
0

(Pdb) step
> .../pdb_jump.py(13)f()
-> result.append(j)

(Pdb) p j
5

(Pdb) continue
> .../pdb_jump.py(12)f()
-> j += n

(Pdb) jump 13
> .../pdb_jump.py(13)f()
-> result.append(j)

(Pdb) p j
10

(Pdb) disable 1

(Pdb) continue
[5, 10, 25, 45, 70]

The program finished and will be restarted
> .../pdb_jump.py(7)<module>()
-> def f(n):
(Pdb)

Jump Back

Jumps can also move the program execution to a statement that has already been executed, to run it again. Here, the value of j is incremented an extra time, so the numbers in the result sequence are all larger than they would otherwise be.

$ python -m pdb pdb_jump.py
> .../pdb_jump.py(7)<module>()
-> def f(n):
(Pdb) break 13
Breakpoint 1 at .../pdb_jump.py:13

(Pdb) continue
> .../pdb_jump.py(13)f()
-> result.append(j)

(Pdb) p j
5

(Pdb) jump 12
> .../pdb_jump.py(12)f()
-> j += n

(Pdb) continue
> .../pdb_jump.py(13)f()
-> result.append(j)

(Pdb) p j
10

(Pdb) disable 1

(Pdb) continue
[10, 20, 35, 55, 80]

The program finished and will be restarted
> .../pdb_jump.py(7)<module>()
-> def f(n):
(Pdb)

Illegal Jumps

Jumping in and out of certain flow control statements is dangerous or undefined, and therefore prevented by the debugger.

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#!/usr/bin/env python
# encoding: utf-8
#
# Copyright (c) 2010 Doug Hellmann.  All rights reserved.
#

def f(n):
    if n < 0:
        raise ValueError('Invalid n: %s' % n)
    result = []
    j = 0
    for i in range(n):
        j = i * n + j
        j += n
        result.append(j)
    return result


if __name__ == '__main__':
    try:
        print f(5)
    finally:
        print 'Always printed'

    try:
        print f(-5)
    except:
        print 'There was an error'
    else:
        print 'There was no error'

    print 'Last statement'

You can jump into a function, but if you do the arguments are not defined and the code is unlikely to work.

$ python -m pdb pdb_no_jump.py
> .../pdb_no_jump.py(7)<module>()
-> def f(n):
(Pdb) break 21
Breakpoint 1 at .../pdb_no_jump.py:21

(Pdb) jump 8
> .../pdb_no_jump.py(8)<module>()
-> if n < 0:

(Pdb) p n
*** NameError: NameError("name 'n' is not defined",)

(Pdb) args

(Pdb)

You cannot jump into the middle of a block such as a for loop or try:except statement.

$ python -m pdb pdb_no_jump.py
> .../pdb_no_jump.py(7)<module>()
-> def f(n):
(Pdb) break 21
Breakpoint 1 at .../pdb_no_jump.py:21

(Pdb) continue
> .../pdb_no_jump.py(21)<module>()
-> print f(5)

(Pdb) jump 26
*** Jump failed: can't jump into the middle of a block

(Pdb)

The code in a finally block must all be executed, so you cannot jump out of the block.

$ python -m pdb pdb_no_jump.py
> .../pdb_no_jump.py(7)<module>()
-> def f(n):
(Pdb) break 23
Breakpoint 1 at .../pdb_no_jump.py:23

(Pdb) continue
[5, 15, 30, 50, 75]
> .../pdb_no_jump.py(23)<module>()
-> print 'Always printed'

(Pdb) jump 25
*** Jump failed: can't jump into or out of a 'finally' block

(Pdb)

And the most basic restriction is that jumping is constrained to the bottom frame on the call stack. If you move up the stack to examine variables, you cannot change the execution flow at that point.

$ python -m pdb pdb_no_jump.py
> .../pdb_no_jump.py(7)<module>()
-> def f(n):
(Pdb) break 11
Breakpoint 1 at .../pdb_no_jump.py:11

(Pdb) continue
> .../pdb_no_jump.py(11)f()
-> j = 0

(Pdb) where
  /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/bdb.py(379)run()
-> exec cmd in globals, locals
  <string>(1)<module>()
  .../pdb_no_jump.py(21)<module>()
-> print f(5)
> .../pdb_no_jump.py(11)f()
-> j = 0

(Pdb) up
> .../pdb_no_jump.py(21)<module>()
-> print f(5)

(Pdb) jump 25
*** You can only jump within the bottom frame

(Pdb)

Restarting Your Program

When the debugger reaches the end of your program, it automatically starts it over, but you can also restart it explicitly without leaving the debugger and losing your breakpoints or other settings.

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#!/usr/bin/env python
# encoding: utf-8
#
# Copyright (c) 2010 Doug Hellmann.  All rights reserved.
#

import sys

def f():
    print 'Command line args:', sys.argv
    return

if __name__ == '__main__':
    f()

Running the above program to completion within the debugger prints the name of the script file, since no other arguments were given on the command line.

$ python -m pdb pdb_run.py
> .../pdb_run.py(7)<module>()
-> import sys
(Pdb) continue

Command line args: ['pdb_run.py']
The program finished and will be restarted
> .../pdb_run.py(7)<module>()
-> import sys

(Pdb)

The program can be restarted using run. Arguments passed to run are parsed with shlex and passed to the program as though they were command line arguments, so you can restart the program with different settings.

(Pdb) run a b c "this is a long value"
Restarting pdb_run.py with arguments:
        a b c this is a long value
> .../pdb_run.py(7)<module>()
-> import sys

(Pdb) continue
Command line args: ['pdb_run.py', 'a', 'b', 'c', 'this is a long value']
The program finished and will be restarted
> .../pdb_run.py(7)<module>()
-> import sys

(Pdb)

run can also be used at any other point in processing to restart the program.

$ python -m pdb pdb_run.py
> .../pdb_run.py(7)<module>()
-> import sys
(Pdb) break 10
Breakpoint 1 at .../pdb_run.py:10

(Pdb) continue
> .../pdb_run.py(10)f()
-> print 'Command line args:', sys.argv

(Pdb) run one two three
Restarting pdb_run.py with arguments:
        one two three
> .../pdb_run.py(7)<module>()
-> import sys

(Pdb)

Customizing the Debugger with Aliases

You can avoid typing complex commands repeatedly by using alias to define a shortcut. Alias expansion is applied to the first word of each command. The body of the alias can consist of any command that is legal to type at the debugger prompt, including other debugger commands and pure Python expressions. Recursion is allowed in alias definitions, so one alias can even invoke another.

$ python -m pdb pdb_function_arguments.py
> .../pdb_function_arguments.py(7)<module>()
-> import pdb
(Pdb) break 10
Breakpoint 1 at .../pdb_function_arguments.py:10

(Pdb) continue
> .../pdb_function_arguments.py(10)recursive_function()
-> if n > 0:

(Pdb) pp locals().keys()
['output', 'n']

(Pdb) alias pl pp locals().keys()

(Pdb) pl
['output', 'n']

Running alias without any arguments shows the list of defined aliases. A single argument is assumed to be the name of an alias, and its definition is printed.

(Pdb) alias
pl = pp locals().keys()

(Pdb) alias pl
pl = pp locals().keys()
(Pdb)

Arguments to the alias are referenced using %n where n is replaced with a number indicating the position of the argument, starting with 1. To consume all of the arguments, use %*.

$ python -m pdb pdb_function_arguments.py
> .../pdb_function_arguments.py(7)<module>()
-> import pdb
(Pdb) alias ph !help(%1)

(Pdb) ph locals
Help on built-in function locals in module __builtin__:

locals(...)
    locals() -> dictionary

    Update and return a dictionary containing the current scope's local variables.

Clear the definition of an alias with unalias.

(Pdb) unalias ph

(Pdb) ph locals
*** SyntaxError: invalid syntax (<stdin>, line 1)

(Pdb)

Saving Configuration Settings

Debugging a program involves a lot of repetition; running the code, observing the output, adjusting the code or inputs, and running it again. pdb attempts to cut down on the amount of repetition you need to use to control the debugging experience, to let you concentrate on your code instead of the debugger. To help reduce the number of times you issue the same commands to the debugger, pdb lets you save configuration using text files read and interpreted on startup.

The file ~/.pdbrc is read first, allowing you to set global personal preferences for all of your debugging sessions. Then ./.pdbrc is read from the current working directory, so you can set local preferences for a particular project.

$ cat ~/.pdbrc
# Show python help
alias ph !help(%1)
# Overridden alias
alias redefined p 'home definition'

$ cat .pdbrc
# Breakpoints
break 10
# Overridden alias
alias redefined p 'local definition'

$ python -m pdb pdb_function_arguments.py
Breakpoint 1 at .../pdb_function_arguments.py:10
> .../pdb_function_arguments.py(7)<module>()
-> import pdb
(Pdb) alias
ph = !help(%1)
redefined = p 'local definition'

(Pdb) break
Num Type         Disp Enb   Where
1   breakpoint   keep yes   at .../pdb_function_arguments.py:10

(Pdb)

Any configuration commands that can be typed at the debugger prompt can be saved in one of the startup files, but most commands that control the execution (continue, jump, etc.) cannot. The exception is run, which means you can set the command line arguments for a debugging session in ./.pdbrc so they are consistent across several runs.

See also

pdb
The standard library documentation for this module.
readline
Interactive prompt editing library.
cmd
Build interactive programs.
shlex
Shell command line parsing.

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