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. 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 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.