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The Python Debugger Pdb
=======================
To use the debugger in its simplest form:
>>> import pdb
>>> pdb.run('<a statement>')
The debugger's prompt is '(Pdb) '. This will stop in the first
function call in <a statement>.
Alternatively, if a statement terminated with an unhandled exception,
you can use pdb's post-mortem facility to inspect the contents of the
traceback:
>>> <a statement>
<exception traceback>
>>> import pdb
>>> pdb.pm()
The commands recognized by the debugger are listed in the next
section. Most can be abbreviated as indicated; e.g., h(elp) means
that 'help' can be typed as 'h' or 'help' (but not as 'he' or 'hel',
nor as 'H' or 'Help' or 'HELP'). Optional arguments are enclosed in
square brackets.
A blank line repeats the previous command literally, except for
'list', where it lists the next 11 lines.
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 variables. When an exception
occurs in such a statement, the exception name is printed but the
debugger's state is not changed.
The debugger supports aliases, which can save typing. And aliases can
have parameters (see the alias help entry) which allows one a certain
level of adaptability to the context under examination.
Multiple commands may be entered on a single line, separated by the
pair ';;'. No intelligence is applied to separating the commands; the
input is split at the first ';;', even if it is in the middle of a
quoted string.
If a file ".pdbrc" exists in your 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 overriden by the local file.
Aside from aliases, the debugger is not directly programmable; but it
is implemented as a class from which you can derive your own debugger
class, which you can make as fancy as you like.
Debugger commands
=================
h(elp)
Without argument, print the list of available commands. With
a command name as argument, print help about that command
(this is currently not implemented).
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 filename:line number argument, set a break there. If
filename is omitted, use the current file. With a function
name, set a break at the first executable line of that
function. Without argument, list all breaks. Each breakpoint
is assigned a number to which all the other breakpoint
commands refer.
The condition argument, if present, is a string which must
evaluate to true in order for the breakpoint to be honored.
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) [bpnumber [bpnumber ...] ]
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.
s(tep)
Execute the current line, stop at the first possible occasion
(either in a function that is called or in the current function).
n(ext)
Continue execution until the next line in the current function
is reached or it returns.
unt(il)
Continue execution until the line with a number greater than the
current one is reached or until the current frame returns.
r(eturn)
Continue execution until the current function returns.
run [args...]
Restart the debugged python program. If a string is supplied it is
splitted 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".
c(ont(inue))
Continue execution, only stop when a breakpoint is encountered.
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 starting at that line.
With two arguments, list the given range;
if the second argument is less than the first, it is a count.
a(rgs)
Print the argument list of the current function.
p expression
Print the value of the expression.
(!) 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
assign to a global variable you must always prefix the command
with a 'global' command, e.g.:
(Pdb) global list_options; list_options = ['-l']
(Pdb)
whatis arg
Prints the type of the argument.
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 name is given, all
aliases are listed.
Aliases may be nested and can contain anything that can be
legally typed at the pdb prompt. Note! You *can* override
internal pdb commands with aliases! Those internal commands
are 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):
#Print instance variables (usage "pi classInst")
alias pi for k in %1.__dict__.keys(): print "%1.",k,"=",%1.__dict__[k]
#Print instance variables in self
alias ps pi self
unalias name
Deletes the specified alias.
q(uit)
Quit from the debugger.
The program being executed is aborted.