26.2. pdb — The Python Debugger — Python 2.7.18 documentation

26.2. pdb — The Python Debugger — Python 2.7.18 documentation
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Python 2.7.18 documentation
The Python Standard Library
26.
Debugging and Profiling
26.2.
pdb
— The Python Debugger
Source code:
Lib/pdb.py
The module
pdb
defines an interactive source code debugger for Python
programs. It supports setting (conditional) breakpoints and single stepping at
the source line level, inspection of stack frames, source code listing, and
evaluation of arbitrary Python code in the context of any stack frame. It also
supports post-mortem debugging and can be called under program control.
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
The debugger’s prompt is
(Pdb)
. Typical usage to run a program under control
of the debugger is:
>>>
import
pdb
>>>
import
mymodule
>>>
pdb
run
'mymodule.test()'
> (0)?()
(Pdb) continue
> (1)?()
(Pdb) continue
NameError: 'spam'
> (1)?()
(Pdb)
pdb.py
can also be invoked as a script to debug other scripts. For
example:
python
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.
New in version 2.4:
Restarting post-mortem behavior added.
The typical usage to break into the debugger from a running program is to
insert
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
command.
The typical usage to inspect a crashed program is:
>>>
import
pdb
>>>
import
mymodule
>>>
mymodule
test
()
Traceback (most recent call last):
File
""
, line
, in

File
"./mymodule.py"
, line
, in
test
test2
()
File
"./mymodule.py"
, line
, in
test2
spam
NameError
spam
>>>
pdb
pm
()
> ./mymodule.py(3)test2()
-> print spam
(Pdb)
The module defines the following functions; each enters the debugger in a
slightly different way:
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
...
Call the
function
(a function or method object, not a string) with the given
arguments. When
runcall()
returns, it returns whatever the function call
returned. The debugger prompt appears as soon as the function is entered.
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
Enter post-mortem debugging of the given
traceback
object. If no
traceback
is given, it uses the one of the exception that is currently
being handled (an exception must be being handled if the default is to be
used).
pdb.
pm
Enter post-mortem debugging of the traceback found in
sys.last_traceback
The
run*
functions and
set_trace()
are aliases for instantiating the
Pdb
class and calling the method of the same name. If you want to
access further features, you have to do this yourself:
class
pdb.
Pdb
completekey='tab'
stdin=None
stdout=None
skip=None
Pdb
is the debugger class.
The
completekey
stdin
and
stdout
arguments are passed to the
underlying
cmd.Cmd
class; see the description there.
The
skip
argument, if given, must be an iterable of glob-style module name
patterns. The debugger will not step into frames that originate in a module
that matches one of these patterns.
Example call to enable tracing with
skip
import
pdb
pdb
Pdb
skip
'django.*'
])
set_trace
()
New in version 2.7:
The
skip
argument.
run
statement
globals
locals
runeval
expression
globals
locals
runcall
function
argument
...
set_trace
See the documentation for the functions explained above.
26.3.
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
or
help
can be used to enter the help command (but not
he
or
hel
, nor
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 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.
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) [
filename:lineno
bpnumber
bpnumber …
]]
With a
filename:lineno
argument, clear all the breakpoints at this line.
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.
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
com
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.
New in version 2.5.
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.
New in version 2.6.
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.
expression
Evaluate the
expression
in the current context and print its value.
Note
can also be used, but is not a debugger command — this executes the
Python
statement.
pp
expression
Like the
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
in
1.
__dict__
keys
():
"%1."
"="
1.
__dict__
#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”.
New in version 2.6.
q(uit)
Quit from the debugger. The program being executed is aborted.
Footnotes
Whether a frame is considered to originate in a certain module
is determined by the
__name__
in the frame globals.
Table of Contents
26.2.
pdb
— The Python Debugger
26.3. Debugger Commands
Previous topic
26.1.
bdb
— Debugger framework
Next topic
26.4.
The Python Profilers
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Python 2.7.18 documentation
The Python Standard Library
26.
Debugging and Profiling
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