9.1 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.

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 an newer frame).

u(p)

Move the current frame one level up in the stack trace (to a 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.

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 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.)

r(eturn)

Continue execution until the current function returns.

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 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.)

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):

#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.

[!]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)

q(uit)

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

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