[Python Std Library] pdb — The Python Debugger

Source From Here
Preface
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 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()')
> (0)?()
(Pdb) continue
> (1)?()
(Pdb) continue
NameError: 'spam'
> (1)?()
(Pdb)

pdb.py can also be invoked as a script to debug other scripts. For example, if we want to test Mock.py
- Mock.py

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1. #!/usr/bin/env python  
2. import pdb, os  
3. import unittest  
4.   
5. class John:  
6.     def __init__(self):  
7.         self.name = "John"  
8.   
9.     def test(self):  
10.         raise Exception("FET", "Test")  
11.   
12. if (__name__ == '__main__'):  
13.     j = John()  
14.     j.test()  

Then we can use below command to trigger pdb while exception occurs:

$ python -m pdb Mock.py
> /root/Mock.py(2)()
-> import pdb, os
(Pdb) n # Continue execution until the next line in the current function is reached or it returns.
> /root/Mock.py(3)()
-> import unittest
(Pdb) c # Continue execution, only stop when a breakpoint is encountered.
Traceback (most recent call last):
...
Exception: ('FET', 'Test')
Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program
> /root/Mock.py(18)test()
-> raise Exception("FET", "Test")
(Pdb) q # Quit from the debugger. The program being executed is aborted.

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.

The typical usage to break into the debugger from a running program is to insert:

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1. import pdb; pdb.set_trace()  

at the location you want to break into the debugger. You can then step through the code following this statement, and continue running without the debugger using the c command.

The typical usage to inspect a crashed program is:

>>> import pdb, Mock
>>> j = Mock.John()
>>> j.test()
Testing...
Raise Exception...
Traceback (most recent call last):
File "", line 1, in
File "Mock.py", line 22, in test
raise Exception("FET", "Test")
Exception: ('FET', 'Test')
>>> pdb.pm()
> /root/Mock.py(22)test()
-> raise Exception("FET", "Test")
(Pdb) <-- console="" enter="" font="" interactive="" pdb="">

The module defines the following functions; each enters the debugger in a slightly different way:
- pdb.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 localsarguments specify the environment in which the code is executed; by default the dictionary of the module __main__ is used. (See the explanation of theexec statement or the eval() built-in function.)

- pdb.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().

- pdb.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.

- pdb.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).

- pdb.post_mortem([traceback])

Enter post-mortem debugging of the given traceback object. If no traceback is given, it uses the one of the exception that is currently being handled (an exception must be being handled if the default is to be used).

- pdb.pm()

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

The run* functions and set_trace() are aliases for instantiating the Pdb class and calling the method of the same name. If you want to access further features, you have to do this yourself:
- class pdb.Pdb(completekey='tab', stdin=None, stdout=None, skip=None)

Pdb is the debugger class.

The completekey, stdin and stdout arguments are passed to the underlying cmd.Cmd class; see the description there.

The skip argument, if given, must be an iterable of glob-style module name patterns. The debugger will not step into frames that originate in a module that matches one of these patterns. [1]

Example call to enable tracing with skip:

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1. import pdb; pdb.Pdb(skip=['django.*']).set_trace()  

Debugger Commands
The debugger recognizes the following commands. Most commands can be abbreviated to one or two letters; e.g. h(elp) means that either h or help can be used to enter the help command (but not he or hel, nor H or Help or HELP). Arguments to commands must be separated by whitespace (spaces or tabs). Optional arguments are enclosed in square brackets ([]) in the command syntax; the square brackets must not be typed. Alternatives in the command syntax are separated by a vertical bar (|).

Entering a blank line repeats the last command entered. Exception: if the last command was a list command, the next 11 lines are listed.

Commands that the debugger doesn’t recognize are assumed to be Python statements and are executed in the context of the program being debugged. Python statements can also be prefixed with an exclamation point (!). This is a powerful way to inspect the program being debugged; it is even possible to change a variable or call a function. When an exception occurs in such a statement, the exception name is printed but the debugger’s state is not changed.

Multiple commands may be entered on a single line, separated by ;;. (A single ; is not used as it is the separator for multiple commands in a line that is passed to the Python parser.) No intelligence is applied to separating the commands; the input is split at the first ;; pair, even if it is in the middle of a quoted string.

The debugger supports aliases. Aliases can have parameters which allows one a certain level of adaptability to the context under examination. For more alias usage, you can refer to Customizing the Debugger with Aliases.

If a file .pdbrc exists in the user’s home directory or in the current directory, it is read in and executed as if it had been typed at the debugger prompt. This is particularly useful for aliases. If both files exist, the one in the home directory is read first and aliases defined there can be overridden by the local file.

- h(elp) [command]

Without argument, print the list of available commands. With a command as argument, print help about that command. help pdb displays the full documentation file; if the environment variable PAGER is defined, the file is piped through that command instead. Since the command argument must be an identifier, help exec must be entered to get help on the ! command.

- w(here)

Print a stack trace, with the most recent frame at the bottom. An arrow indicates the current frame, which determines the context of most commands.

- d(own)

Move the current frame one level down in the stack trace (to a newer frame).

- u(p)

Move the current frame one level up in the stack trace (to an older frame).

- b(reak) [[filename:]lineno | function[, condition]]

With a lineno argument, set a break there in the current file. With a function argument, set a break at the first executable statement within that function. The line number may be prefixed with a filename and a colon, to specify a breakpoint in another file (probably one that hasn’t been loaded yet). The file is searched on sys.path. Note that each breakpoint is assigned a number to which all the other breakpoint commands refer.

If a second argument is present, it is an expression which must evaluate to true before the breakpoint is honored.

Without argument, list all breaks, including for each breakpoint, the number of times that breakpoint has been hit, the current ignore count, and the associated condition if any. Below is the usage example:

(Pdb) list # Show code with line number
12
13 class John:
14 def __init__(self):
15 self.name = "John"
16
17 def otherFun(self):
18 print "Entering otherFun..."
19 print "Exit otherFun..."
20
21 def test(self):
22 #print "Enter pdb..."
(Pdb) break 17 # Set breakpoint in line number 17
Breakpoint 1 at /root/Mock.py:17
(Pdb) c # Continue the program
> /root/Mock.py(17)John()
-> def otherFun(self):
(Pdb)

- tbreak [[filename:]lineno | function[, condition]]

Temporary breakpoint, which is removed automatically when it is first hit. The arguments are the same as break.

- cl(ear) [filename:lineno | bpnumber [bpnumber ...]]

With a filename:lineno argument, clear all the breakpoints at this line. With a space separated list of breakpoint numbers, clear those breakpoints. Without argument, clear all breaks (but first ask confirmation).

- disable [bpnumber [bpnumber ...]]

Disables the breakpoints given as a space separated list of breakpoint numbers. Disabling a breakpoint means it cannot cause the program to stop execution, but unlike clearing a breakpoint, it remains in the list of breakpoints and can be (re-)enabled.

- enable [bpnumber [bpnumber ...]]

Enables the breakpoints specified.

- ignore bpnumber [count]

Sets the ignore count for the given breakpoint number. If count is omitted, the ignore count is set to 0. A breakpoint becomes active when the ignore count is zero. When non-zero, the count is decremented each time the breakpoint is reached and the breakpoint is not disabled and any associated condition evaluates to true.

- condition bpnumber [condition]

Condition is an expression which must evaluate to true before the breakpoint is honored. If condition is absent, any existing condition is removed; i.e., the breakpoint is made unconditional.

- commands [bpnumber]

Specify a list of commands for breakpoint number bpnumber. The commands themselves appear on the following lines. Type a line containing just ‘end’ to terminate the commands. An example:

(Pdb) commands 1 # Setup command for breakpoint num 1
(com) print some_variable
(com) end # Stop command definition for breakpoint num1
(Pdb)

To remove all commands from a breakpoint, type commands and follow it immediately with end; that is, give no commands.

With no bpnumber argument, commands refers to the last breakpoint set.

You can use breakpoint commands to start your program up again. Simply use the continue command, or step, or any other command that resumes execution.

Specifying any command resuming execution (currently continue, step, next, return, jump, quit and their abbreviations) terminates the command list (as if that command was immediately followed by end). This is because any time you resume execution (even with a simple next or step), you may encounter another breakpoint–which could have its own command list, leading to ambiguities about which list to execute.

If you use the ‘silent’ command in the command list, the usual message about stopping at a breakpoint is not printed. This may be desirable for breakpoints that are to print a specific message and then continue. If none of the other commands print anything, you see no sign that the breakpoint was reached.

- s(tep)

Execute the current line, stop at the first possible occasion (either in a function that is called or on the next line in the current function).

- n(ext)

Continue execution until the next line in the current function is reached or it returns. (The difference between next and step is that step stops inside a called function, while next executes called functions at (nearly) full speed, only stopping at the next line in the current function.)

- unt(il)

Continue execution until the line with the line number greater than the current one is reached or when returning from current frame.

- r(eturn)

Continue execution until the current function returns.

- c(ont(inue))

Continue execution, only stop when a breakpoint is encountered.

- j(ump) lineno

Set the next line that will be executed. Only available in the bottom-most frame. This lets you jump back and execute code again, or jump forward to skip code that you don’t want to run.

It should be noted that not all jumps are allowed — for instance it is not possible to jump into the middle of a for loop or out of a finally clause.

- l(ist) [first[, last]]

List source code for the current file. Without arguments, list 11 lines around the current line or continue the previous listing. With one argument, list 11 lines around at that line. With two arguments, list the given range; if the second argument is less than the first, it is interpreted as a count.

- a(rgs)

Print the argument list of the current function.

- p expression

Evaluate the expression in the current context and print its value.

Note: print can also be used, but is not a debugger command — this executes the Python print statement.

- pp expression

Like the p command, except the value of the expression is pretty-printed using the pprint module.

- alias [name [command]]

Creates an alias called name that executes command. The command must not be enclosed in quotes. Replaceable parameters can be indicated by %1, %2, and so on, while %* is replaced by all the parameters. If no command is given, the current alias for name is shown. If no arguments are given, all aliases are listed.

Aliases may be nested and can contain anything that can be legally typed at the pdb prompt. Note that internal pdb commands can be overridden by aliases.Such a command is then hidden until the alias is removed. Aliasing is recursively applied to the first word of the command line; all other words in the line are left alone.

As an example, here are two useful aliases (especially when placed in the .pdbrc file):

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1. #Print instance variables (usage "pi classInst")  
2. alias pi for k in %1.__dict__.keys(): print "%1.",k,"=",%1.__dict__[k]  
3. #Print instance variables in self  
4. alias ps pi self  

- unalias name

Deletes the specified alias.

- [!]statement

Execute the (one-line) statement in the context of the current stack frame. The exclamation point can be omitted unless the first word of the statement resembles a debugger command. To set a global variable, you can prefix the assignment command with a global command on the same line, e.g.:

(Pdb) global list_options; list_options = ['-l']
(Pdb)

- run [args ...]

Restart the debugged Python program. If an argument is supplied, it is split with “shlex” and the result is used as the new sys.argv. History, breakpoints, actions and debugger options are preserved. “restart” is an alias for “run”.

- q(uit)

Quit from the debugger. The program being executed is aborted.

Supplement
* pdb – Interactive Debugger

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...