26.2. pdb — Python的调试器

源代码： Lib/pdb.py

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pdb 模块定义了一个交互式源代码调试器，用于 Python 程序。它支持在源码行间设置（有条件的）断点和单步执行，检视堆栈帧，列出源码列表，以及在任何堆栈帧的上下文中运行任意 Python 代码。它还支持事后调试，可以在程序控制下调用。

The debugger is extensible — it is actually defined as the class Pdb. This is currently undocumented but easily understood by reading the source. The extension interface uses the modules bdb and cmd.

调试器的提示符是 (Pdb)。在调试器的控制下运行程序的典型用法是:

>>> import pdb
>>> import mymodule
>>> pdb.run('mymodule.test()')
> <string>(0)?()
(Pdb) continue
> <string>(1)?()
(Pdb) continue
NameError: 'spam'
> <string>(1)?()
(Pdb)

也可以将 pdb.py 作为脚本调用来调试其他脚本。例如:

python -m pdb myscript.py

When invoked as a script, pdb will automatically enter post-mortem debugging if the program being debugged exits abnormally. After post-mortem debugging (or after normal exit of the program), pdb will restart the program. Automatic restarting preserves pdb’s state (such as breakpoints) and in most cases is more useful than quitting the debugger upon program’s exit.

2.4 新版功能: Restarting post-mortem behavior added.

从正在运行的程序进入调试器的典型用法是插入

import pdb; pdb.set_trace()

at the location you want to break into the debugger. You can then step through the code following this statement, and continue running without the debugger using the c command.

检查已崩溃程序的典型用法是:

>>> import pdb
>>> import mymodule
>>> mymodule.test()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "./mymodule.py", line 4, in test
    test2()
  File "./mymodule.py", line 3, in test2
    print spam
NameError: spam
>>> pdb.pm()
> ./mymodule.py(3)test2()
-> print spam
(Pdb)

本模块定义了下列函数，每个函数进入调试器的方式略有不同：

pdb.run(statement[, globals[, locals]])

Execute the statement (given as a string) under debugger control. The debugger prompt appears before any code is executed; you can set breakpoints and type continue, or you can step through the statement using step or next (all these commands are explained below). The optional globals and locals arguments specify the environment in which the code is executed; by default the dictionary of the module __main__ is used. (See the explanation of the exec statement or the eval() built-in function.)

pdb.runeval(expression[, globals[, locals]])

Evaluate the expression (given as a string) under debugger control. When runeval() returns, it returns the value of the expression. Otherwise this function is similar to run().

pdb.runcall(function[, argument, ...])

使用给定的参数调用 function （以函数或方法对象的形式提供，不能是字符串）。runcall() 返回的是所调用函数的返回值。调试器提示符将在进入函数后立即出现。

pdb.set_trace()

Enter the debugger at the calling stack frame. This is useful to hard-code a breakpoint at a given point in a program, even if the code is not otherwise being debugged (e.g. when an assertion fails).

pdb.post_mortem([traceback])

进入 traceback 对象的事后调试。如果没有给定 traceback，默认使用当前正在处理的异常之一（默认时，必须存在正在处理的异常）。

pdb.pm()

在 sys.last_traceback 中查找 traceback，并进入其事后调试。

run* 函数和 set_trace() 都是别名，用于实例化 Pdb 类和调用同名方法。如果要使用其他功能，则必须自己执行以下操作：

class pdb.Pdb(completekey='tab', stdin=None, stdout=None, skip=None)

Pdb 是调试器类。

completekey、stdin 和 stdout 参数都会传递给底层的 cmd.Cmd 类，请参考相应的描述。

如果给出 skip 参数，则它必须是一个迭代器，可以迭代出 glob-style 样式的模块名称。如果遇到匹配上述样式的模块，调试器将不会进入来自该模块的堆栈帧。 1

启用跟踪且带有 skip 参数的调用示范:

import pdb; pdb.Pdb(skip=['django.*']).set_trace()

2.7 新版功能: skip 参数。

run(statement[, globals[, locals]])

runeval(expression[, globals[, locals]])

runcall(function[, argument, ...])

set_trace()

请参阅上文解释同名函数的文档。

26.3. 调试器命令

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.

调试器无法识别的命令将被认为是 Python 语句，并在正在调试的程序的上下文中执行。Python 语句也可以用感叹号 (!) 作为前缀。这是检查正在调试的程序的强大方法，甚至可以修改变量或调用函数。当此类语句发生异常，将打印异常名称，但调试器的状态不会改变。

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)

打印堆栈回溯，最新一帧在底部。有一个箭头指向当前帧，该帧决定了大多数命令的上下文。

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 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.

如果第二个参数存在，它应该是一个表达式，且它的计算值为 true 时断点才起作用。

如果不带参数执行，将列出所有中断，包括每个断点、命中该断点的次数、当前的忽略次数以及关联的条件（如果有）。

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) [filename:lineno | bpnumber [bpnumber …]]

如果参数是 filename:lineno，则清除此行上的所有断点。如果参数是空格分隔的断点编号列表，则清除这些断点。如果不带参数，则清除所有断点（但会先提示确认）。

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.

commands [bpnumber]

Specify a list of commands for breakpoint number bpnumber. The commands themselves appear on the following lines. Type a line containing just ‘end’ to terminate the commands. An example:

(Pdb) commands 1
(com) print some_variable
(com) end
(Pdb)

To remove all commands from a breakpoint, type commands and follow it immediately with end; that is, give no commands.

With no bpnumber argument, commands refers to the last breakpoint set.

You can use breakpoint commands to start your program up again. Simply use the continue command, or step, or any other command that resumes execution.

Specifying any command resuming execution (currently continue, step, next, return, jump, quit and their abbreviations) terminates the command list (as if that command was immediately followed by end). This is because any time you resume execution (even with a simple next or step), you may encounter another breakpoint—which could have its own command list, leading to ambiguities about which list to execute.

If you use the ‘silent’ command in the command list, the usual message about stopping at a breakpoint is not printed. This may be desirable for breakpoints that are to print a specific message and then continue. If none of the other commands print anything, you see no sign that the breakpoint was reached.

2.5 新版功能.

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.)

unt(il)

Continue execution until the line with the line number greater than the current one is reached or when returning from current frame.

2.6 新版功能.

r(eturn)

Continue execution until the current function returns.

c(ont(inue))

Continue execution, only stop when a breakpoint is encountered.

j(ump) lineno

Set the next line that will be executed. Only available in the bottom-most frame. This lets you jump back and execute code again, or jump forward to skip code that you don’t want to run.

It should be noted that not all jumps are allowed — for instance it is not possible to jump into the middle of a for loop or out of a finally clause.

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.

注解

print can also be used, but is not a debugger command — this executes the Python print statement.

pp expression

Like the p command, except the value of the expression is pretty-printed using the pprint module.

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)

run [args …]

Restart the debugged Python program. If an argument is supplied, it is split 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”.

2.6 新版功能.

q(uit)

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

备注

1

Whether a frame is considered to originate in a certain module is determined by the __name__ in the frame globals.