320 lines
11 KiB
TeX
320 lines
11 KiB
TeX
\chapter{The Python Debugger}
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\stmodindex{pdb}
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\index{debugging}
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\setindexsubitem{(in module pdb)}
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The module \code{pdb} defines an interactive source code debugger for
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Python programs. It supports setting
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(conditional) breakpoints and single stepping
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at the source line level, inspection of stack frames, source code
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listing, and evaluation of arbitrary Python code in the context of any
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stack frame. It also supports post-mortem debugging and can be called
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under program control.
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The debugger is extensible --- it is actually defined as a class
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\code{Pdb}. This is currently undocumented but easily understood by
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reading the source. The extension interface uses the (also
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undocumented) modules \code{bdb} and \code{cmd}.
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\ttindex{Pdb}
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\ttindex{bdb}
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\ttindex{cmd}
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A primitive windowing version of the debugger also exists --- this is
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module \code{wdb}, which requires STDWIN (see the chapter on STDWIN
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specific modules).
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\index{stdwin}
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\ttindex{wdb}
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The debugger's prompt is ``\code{(Pdb) }''.
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Typical usage to run a program under control of the debugger is:
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\begin{verbatim}
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>>> import pdb
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>>> import mymodule
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>>> pdb.run('mymodule.test()')
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> <string>(0)?()
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(Pdb) continue
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> <string>(1)?()
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(Pdb) continue
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NameError: 'spam'
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> <string>(1)?()
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(Pdb)
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\end{verbatim}
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%
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\code{pdb.py} can also be invoked as
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a script to debug other scripts. For example:
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\code{python /usr/local/lib/python1.4/pdb.py myscript.py}
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Typical usage to inspect a crashed program is:
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\begin{verbatim}
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>>> import pdb
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>>> import mymodule
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>>> mymodule.test()
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Traceback (innermost last):
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File "<stdin>", line 1, in ?
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File "./mymodule.py", line 4, in test
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test2()
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File "./mymodule.py", line 3, in test2
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print spam
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NameError: spam
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>>> pdb.pm()
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> ./mymodule.py(3)test2()
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-> print spam
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(Pdb)
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\end{verbatim}
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%
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The module defines the following functions; each enters the debugger
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in a slightly different way:
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\begin{funcdesc}{run}{statement\optional{\, globals\optional{\, locals}}}
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Execute the \var{statement} (given as a string) under debugger
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control. The debugger prompt appears before any code is executed; you
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can set breakpoints and type \code{continue}, or you can step through
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the statement using \code{step} or \code{next} (all these commands are
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explained below). The optional \var{globals} and \var{locals}
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arguments specify the environment in which the code is executed; by
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default the dictionary of the module \code{__main__} is used. (See
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the explanation of the \code{exec} statement or the \code{eval()}
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built-in function.)
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\end{funcdesc}
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\begin{funcdesc}{runeval}{expression\optional{\, globals\optional{\, locals}}}
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Evaluate the \var{expression} (given as a a string) under debugger
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control. When \code{runeval()} returns, it returns the value of the
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expression. Otherwise this function is similar to
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\code{run()}.
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\end{funcdesc}
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\begin{funcdesc}{runcall}{function\optional{\, argument\, ...}}
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Call the \var{function} (a function or method object, not a string)
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with the given arguments. When \code{runcall()} returns, it returns
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whatever the function call returned. The debugger prompt appears as
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soon as the function is entered.
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\end{funcdesc}
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\begin{funcdesc}{set_trace}{}
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Enter the debugger at the calling stack frame. This is useful to
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hard-code a breakpoint at a given point in a program, even if the code
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is not otherwise being debugged (e.g. when an assertion fails).
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\end{funcdesc}
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\begin{funcdesc}{post_mortem}{traceback}
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Enter post-mortem debugging of the given \var{traceback} object.
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\end{funcdesc}
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\begin{funcdesc}{pm}{}
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Enter post-mortem debugging of the traceback found in
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\code{sys.last_traceback}.
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\end{funcdesc}
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\section{Debugger Commands}
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The debugger recognizes the following commands. Most commands can be
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abbreviated to one or two letters; e.g. ``\code{h(elp)}'' means that
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either ``\code{h}'' or ``\code{help}'' can be used to enter the help
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command (but not ``\code{he}'' or ``\code{hel}'', nor ``\code{H}'' or
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``\code{Help} or ``\code{HELP}''). Arguments to commands must be
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separated by whitespace (spaces or tabs). Optional arguments are
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enclosed in square brackets (``\code{[]}'') in the command syntax; the
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square brackets must not be typed. Alternatives in the command syntax
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are separated by a vertical bar (``\code{|}'').
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Entering a blank line repeats the last command entered. Exception: if
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the last command was a ``\code{list}'' command, the next 11 lines are
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listed.
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Commands that the debugger doesn't recognize are assumed to be Python
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statements and are executed in the context of the program being
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debugged. Python statements can also be prefixed with an exclamation
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point (``\code{!}''). This is a powerful way to inspect the program
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being debugged; it is even possible to change a variable or call a
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function. When an
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exception occurs in such a statement, the exception name is printed
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but the debugger's state is not changed.
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\begin{description}
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\item[h(elp) \optional{\var{command}}]
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Without argument, print the list of available commands. With a
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\var{command} as argument, print help about that command. \samp{help
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pdb} displays the full documentation file; if the environment variable
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\code{PAGER} is defined, the file is piped through that command
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instead. Since the \var{command} argument must be an identifier,
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\samp{help exec} must be entered to get help on the \samp{!} command.
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\item[w(here)]
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Print a stack trace, with the most recent frame at the bottom. An
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arrow indicates the current frame, which determines the context of
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most commands.
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\item[d(own)]
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Move the current frame one level down in the stack trace
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(to an older frame).
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\item[u(p)]
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Move the current frame one level up in the stack trace
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(to a newer frame).
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\item[b(reak) \optional{\var{lineno}{\Large\code{|}}\var{function}%
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\optional{, \code{'}\var{condition}\code{'}}}]
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With a \var{lineno} argument, set a break there in the current
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file. With a \var{function} argument, set a break at the entry of
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that function. Without argument, list all breaks.
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If a second argument is present, it is a string (included in string
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quotes!) specifying an expression which must evaluate to true before
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the breakpoint is honored.
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\item[cl(ear) \optional{\var{lineno}}]
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With a \var{lineno} argument, clear that break in the current file.
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Without argument, clear all breaks (but first ask confirmation).
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\item[s(tep)]
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Execute the current line, stop at the first possible occasion
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(either in a function that is called or on the next line in the
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current function).
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\item[n(ext)]
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Continue execution until the next line in the current function
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is reached or it returns. (The difference between \code{next} and
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\code{step} is that \code{step} stops inside a called function, while
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\code{next} executes called functions at (nearly) full speed, only
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stopping at the next line in the current function.)
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\item[r(eturn)]
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Continue execution until the current function returns.
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\item[c(ont(inue))]
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Continue execution, only stop when a breakpoint is encountered.
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\item[l(ist) \optional{\var{first\optional{, last}}}]
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List source code for the current file. Without arguments, list 11
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lines around the current line or continue the previous listing. With
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one argument, list 11 lines around at that line. With two arguments,
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list the given range; if the second argument is less than the first,
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it is interpreted as a count.
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\item[a(rgs)]
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Print the argument list of the current function.
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\item[p \var{expression}]
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Evaluate the \var{expression} in the current context and print its
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value. (Note: \code{print} can also be used, but is not a debugger
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command --- this executes the Python \code{print} statement.)
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\item[\optional{!}\var{statement}]
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Execute the (one-line) \var{statement} in the context of
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the current stack frame.
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The exclamation point can be omitted unless the first word
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of the statement resembles a debugger command.
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To set a global variable, you can prefix the assignment
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command with a ``\code{global}'' command on the same line, e.g.:
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\begin{verbatim}
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(Pdb) global list_options; list_options = ['-l']
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(Pdb)
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\end{verbatim}
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%
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\item[q(uit)]
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Quit from the debugger.
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The program being executed is aborted.
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\end{description}
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\section{How It Works}
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Some changes were made to the interpreter:
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\begin{itemize}
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\item \code{sys.settrace(\var{func})} sets the global trace function
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\item there can also a local trace function (see later)
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\end{itemize}
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Trace functions have three arguments: (\var{frame}, \var{event}, \var{arg})
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\begin{description}
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\item[\var{frame}] is the current stack frame
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\item[\var{event}] is a string: \code{'call'}, \code{'line'}, \code{'return'}
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or \code{'exception'}
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\item[\var{arg}] is dependent on the event type
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\end{description}
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The global trace function is invoked (with \var{event} set to
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\code{'call'}) whenever a new local scope is entered; it should return
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a reference to the local trace function to be used that scope, or
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\code{None} if the scope shouldn't be traced.
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The local trace function should return a reference to itself (or to
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another function for further tracing in that scope), or \code{None} to
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turn off tracing in that scope.
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Instance methods are accepted (and very useful!) as trace functions.
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The events have the following meaning:
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\begin{description}
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\item[\code{'call'}]
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A function is called (or some other code block entered). The global
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trace function is called; arg is the argument list to the function;
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the return value specifies the local trace function.
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\item[\code{'line'}]
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The interpreter is about to execute a new line of code (sometimes
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multiple line events on one line exist). The local trace function is
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called; arg in None; the return value specifies the new local trace
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function.
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\item[\code{'return'}]
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A function (or other code block) is about to return. The local trace
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function is called; arg is the value that will be returned. The trace
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function's return value is ignored.
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\item[\code{'exception'}]
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An exception has occurred. The local trace function is called; arg is
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a triple (exception, value, traceback); the return value specifies the
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new local trace function
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\end{description}
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Note that as an exception is propagated down the chain of callers, an
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\code{'exception'} event is generated at each level.
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Stack frame objects have the following read-only attributes:
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\begin{description}
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\item[f_code] the code object being executed
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\item[f_lineno] the current line number (\code{-1} for \code{'call'} events)
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\item[f_back] the stack frame of the caller, or None
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\item[f_locals] dictionary containing local name bindings
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\item[f_globals] dictionary containing global name bindings
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\end{description}
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Code objects have the following read-only attributes:
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\begin{description}
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\item[co_code] the code string
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\item[co_names] the list of names used by the code
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\item[co_consts] the list of (literal) constants used by the code
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\item[co_filename] the filename from which the code was compiled
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\end{description}
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