192 lines
7.3 KiB
Text
192 lines
7.3 KiB
Text
The Python Debugger Pdb
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=======================
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To use the debugger in its simplest form:
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>>> import pdb
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>>> pdb.run('<a statement>')
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The debugger's prompt is '(Pdb) '. This will stop in the first
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function call in <a statement>.
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Alternatively, if a statement terminated with an unhandled exception,
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you can use pdb's post-mortem facility to inspect the contents of the
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traceback:
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>>> <a statement>
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<exception traceback>
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>>> import pdb
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>>> pdb.pm()
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The commands recognized by the debugger are listed in the next
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section. Most can be abbreviated as indicated; e.g., h(elp) means
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that 'help' can be typed as 'h' or 'help' (but not as 'he' or 'hel',
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nor as 'H' or 'Help' or 'HELP'). Optional arguments are enclosed in
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square brackets.
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A blank line repeats the previous command literally, except for
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'list', where it lists the next 11 lines.
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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 ('!'). This is a powerful way to inspect the program being
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debugged; it is even possible to change variables. When an exception
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occurs in such a statement, the exception name is printed but the
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debugger's state is not changed.
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The debugger supports aliases, which can save typing. And aliases can
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have parameters (see the alias help entry) which allows one a certain
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level of adaptability to the context under examination.
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Multiple commands may be entered on a single line, separated by the
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pair ';;'. No intelligence is applied to separating the commands; the
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input is split at the first ';;', even if it is in the middle of a
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quoted string.
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If a file ".pdbrc" exists in your home directory or in the current
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directory, it is read in and executed as if it had been typed at the
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debugger prompt. This is particularly useful for aliases. If both
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files exist, the one in the home directory is read first and aliases
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defined there can be overriden by the local file.
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Aside from aliases, the debugger is not directly programmable; but it
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is implemented as a class from which you can derive your own debugger
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class, which you can make as fancy as you like.
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Debugger commands
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=================
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h(elp)
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Without argument, print the list of available commands. With
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a command name as argument, print help about that command
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(this is currently not implemented).
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w(here)
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Print a stack trace, with the most recent frame at the bottom.
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An arrow indicates the "current frame", which determines the
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context of most commands.
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d(own)
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Move the current frame one level down in the stack trace
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(to a newer frame).
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u(p)
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Move the current frame one level up in the stack trace
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(to an older frame).
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b(reak) [ ([filename:]lineno | function) [, condition] ]
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With a filename:line number argument, set a break there. If
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filename is omitted, use the current file. With a function
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name, set a break at the first executable line of that
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function. Without argument, list all breaks. Each breakpoint
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is assigned a number to which all the other breakpoint
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commands refer.
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The condition argument, if present, is a string which must
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evaluate to true in order for the breakpoint to be honored.
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tbreak [ ([filename:]lineno | function) [, condition] ]
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Temporary breakpoint, which is removed automatically when it
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is first hit. The arguments are the same as break.
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cl(ear) [bpnumber [bpnumber ...] ]
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With a space separated list of breakpoint numbers, clear those
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breakpoints. Without argument, clear all breaks (but first
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ask confirmation).
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disable bpnumber [bpnumber ...]
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Disables the breakpoints given as a space separated list of
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breakpoint numbers. Disabling a breakpoint means it cannot
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cause the program to stop execution, but unlike clearing a
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breakpoint, it remains in the list of breakpoints and can be
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(re-)enabled.
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enable bpnumber [bpnumber ...]
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Enables the breakpoints specified.
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ignore bpnumber count
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Sets the ignore count for the given breakpoint number. If
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count is omitted, the ignore count is set to 0. A breakpoint
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becomes active when the ignore count is zero. When non-zero,
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the count is decremented each time the breakpoint is reached
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and the breakpoint is not disabled and any associated
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condition evaluates to true.
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condition bpnumber condition
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condition is an expression which must evaluate to true before
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the breakpoint is honored. If condition is absent, any
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existing condition is removed; i.e., the breakpoint is made
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unconditional.
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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 in the current function).
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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.
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r(eturn)
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Continue execution until the current function returns.
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c(ont(inue))
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Continue execution, only stop when a breakpoint is encountered.
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l(ist) [first [,last]]
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List source code for the current file.
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Without arguments, list 11 lines around the current line
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or continue the previous listing.
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With one argument, list 11 lines starting at that line.
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With two arguments, list the given range;
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if the second argument is less than the first, it is a count.
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a(rgs)
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Print the argument list of the current function.
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p expression
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Print the value of the expression.
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(!) statement
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Execute the (one-line) statement in the context of the current
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stack frame. The exclamation point can be omitted unless the
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first word of the statement resembles a debugger command. To
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assign to a global variable you must always prefix the command
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with a 'global' command, e.g.:
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(Pdb) global list_options; list_options = ['-l']
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(Pdb)
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whatis arg
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Prints the type of the argument.
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alias [name [command]]
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Creates an alias called 'name' that executes 'command'. The
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command must *not* be enclosed in quotes. Replaceable
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parameters can be indicated by %1, %2, and so on, while %* is
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replaced by all the parameters. If no command is given, the
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current alias for name is shown. If no name is given, all
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aliases are listed.
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Aliases may be nested and can contain anything that can be
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legally typed at the pdb prompt. Note! You *can* override
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internal pdb commands with aliases! Those internal commands
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are then hidden until the alias is removed. Aliasing is
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recursively applied to the first word of the command line; all
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other words in the line are left alone.
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As an example, here are two useful aliases (especially when
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placed in the .pdbrc file):
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#Print instance variables (usage "pi classInst")
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alias pi for k in %1.__dict__.keys(): print "%1.",k,"=",%1.__dict__[k]
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#Print instance variables in self
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alias ps pi self
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unalias name
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Deletes the specified alias.
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q(uit)
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Quit from the debugger.
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The program being executed is aborted.
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