1   Getting started with otrace and oshell

This tutorial introduces the main features of otrace by explaining how to “debug” the demo program, hello_trace.py, included in the distribution.

1.1   Installation

Download the development version of otrace from Github, or the released archive from the Python Package Index The unzipped archive should contain the following files (and some more):

hello_trace.py ordereddict.py otrace.py README.rst setup.py ...

All the code for the otrace module is contained in a single file, otrace.py. (For python 2.6 or earlier, you will also need ordereddict.py.) To use otrace without installing it, just ensure that these files are present in the module load path. If you wish to install otrace, use:

python setup.py install

1.2   Demo program

hello_trace.py is a simple multi-threaded web server using the BaseHTTPServer module. It listens on port 8888 and displays a simple form where the user inputs a number and the server displays the reciprocal of the number. Inputting a zero value will raise an exception, which will be used to illustrate the capabilities of otrace.

All the oshell commands used below are available in the script demo/hello_test.osh. Once you are comfortable with the steps in this tutorial, you can re-run the entire tutorial using the following command (in the otrace console):

source ~~w/demo/hello_test.osh

1.3   help, cd, ls, and view commands

Run hello_trace.py from the command line (user input appears after the prompt “>”). The help command displays all the available oshell commands:

otrace$ ./hello_trace.py
Listening on port 8888
  ***otrace object shell (v0.30.0+)*** (type 'help' for info)
> help
Commands:
alias    cd       cdls     del      dn       edit     exec     help
lock     ls       popd     pr       pushd    pwd      quit     repeat
resume   rm       run      save     set      source   swapd    tag
trace    unpatch  unpickle untag    untrace  up       view

If you omit the command, "pr" is assumed.
Use TAB key for command completion.
Type "help <command>" or "help *" for more info

See http://code.mindmeldr.com/otrace for documentation
globals> help pwd
pwd [-a]                  # Print current working "directory"

-a Print all paths in stack; top first

The pwd command displays the current working directory. The ls command displays directory names and content. The special directory name ~~ refers to the most recent trace context, and is initially /osh/globals. There are also other special directory names of the form ~~<letter>:

> pwd
/osh/globals
> ls ~~
/osh/globals
> ls ~~g
/osh/globals
> ls ~~w
/Users/rsarava/app4/repo/meldr-hg/otrace

The ls options -c, -f, -m, -v can be used to selectively display only the classes, functions/methods, modules, and variables in a directory. Initially we examine the globals directory, which contains three classes, GetHandler, MultiThreadedServer, and OShell:

> ls
BaseHTTPServer      GetHandler          MultiThreadedServer OShell
Page_template       Receive             Request_stats       SocketServer
http_addr           http_port           http_server         logging
send                sleep               sys                 test_fun
trace_shell         traceassert         urlparse
> ls -c
GetHandler          MultiThreadedServer OShell              Receive

The cd command is used to change directories. Switching to the directory of class GetHandler, we note that it supports several methods, many of which are inherited:

> cd GetHandler
osh..GetHandler> ls -f
address_string       date_time_string     do_GET               end_headers
finish               handle               handle_one_request   log_date_time_string
log_error            log_message          log_request          parse_request
send_error           send_header          send_response        setup
version_string

The ls options -.. can be used to exclude inherited attributes of a class, and we note that GetHandler has just one method of its own:

osh..GetHandler> ls -f -..
do_GET

We can examine the source code for the Receive.respond method using the view command with the -i (inline-display) option:

osh..GetHandler> cd ..
globals> cd Receive
osh..Receive> ls
respond
osh..Receive> view -i respond
def respond(self, request):
    # Respond to request by computing reciprocal and returning response string

    # Diagnostic print (initially commented out)
    ##if self.value <= 0.001:
    ##    print "Client address", request.client_address

    # Trace assertion (initially commented out)
    ##otrace.traceassert(self.value > 0.001, label="num_check")

    # Compute reciprocal of number
    response = "The reciprocal of %s is %s" % (self.value, 1.0/self.value)
    return response

1.4   pr and exec commands

The pr command prints out the value of a python expression. It is the default command, and is assumed if no command is recognized. So python expressions can usually be evaluated directly:

> pwd
/osh/globals
> pr Request_stats
{'count': 0, 'path': ''}
> Request_stats["count"]
0
> set safe_mode False
safe_mode = False
> abs(Request_stats["count"] - 1)
1

To prevent inadvertent modification of a running program through function calls, parentheses are not allowed in pr expressions by default. Setting the safe_mode parameter to False allows their use.

The exec command executes a python statement, like assignment or import. The prefix ! may be used instead of exec. safe_mode must be False to use exec:

> !Request_stats["count"] = 2

1.5   trace command

The trace command is used to trace functions and methods. Without any options, it simply traces exceptions. The -c <condition> option, where <condition> may be call, return, or all, may be used to trace function/method calls, returns, or both. <condition> may also be argname1.comp1==value1,argname2!=value2,... to trace on argument value matching (values with commas/spaces must be quoted; the special argument name return may also be used). Without any arguments, the trace command displays currently traced names. Next, we initiate tracing on the respond method using the trace command:

globals> cd ~~g
globals> cd Receive
osh..Receive> trace respond
Tracing Receive.respond
osh..Receive> trace
Receive.respond

Now we are ready to load the URL http://localhost:8888 in the browser, and enter numbers. Instead of using the browser, in this demo we will use the function submit that simulates browser input from the user. The command “submit(22)” would be equivalent to the user entering 22. A log message is generated for each value, and the zero input value triggers a ZeroDivisionError exception in the respond method. In the exception backtrace shown below, note the additional methods otrace_wrapped and otrace_function_call between do_GET and respond. These are inserted by otrace for tracing:

osh..Receive> submit(3)
rootW path=/?number=3
  <span>The reciprocal of 3.0 is 0.333333333333</span>
osh..Receive> submit(0)
rootW path=/?number=0
Receive.respond:ex-ZeroDivisionError:05-08-45
rootE ERROR: float division by zero
Server error:
Traceback (most recent call last):
  File "./hello_trace.py", line 76, in do_GET
    resp = Page_template % recv.respond(self)
  File "/Users/rsarava/app4/repo/mitotic/otrace/otrace.py", line 4601, in otrace_wrapped
    return cls.otrace_function_call(func_info, *args, **kwargs)
  File "/Users/rsarava/app4/repo/mitotic/otrace/otrace.py", line 4373, in otrace_function_call
    return_value = info.fn(*args, **kwargs)
  File "./hello_trace.py", line 104, in respond
    response = "The reciprocal of %s is %s" % (self.value, 1.0/self.value)
ZeroDivisionError: float division by zero

When a trace condition occurs, like an exception in a traced function or method, a trace id GetHandler.respond:ex-ZeroDivisionError:05-08-45 is generated and displayed, as shown above. Also, the default action of the trace command is to create a new virtual directory /osh/recent/exceptions/GetHandler.respond/ex-ZeroDivisionError/05-08-45 to hold the trace context for the event. The shorthand notation ~~ can be used to display the most recent trace context:

osh..Receive> ls ~~
/osh/recent/exceptions/Receive.respond/ex-ZeroDivisionError/05-08-45
osh..Receive> cd ~~

The trace context contains information about the function like argument values and the call stack.:

Receive..08-45> ls
__trc   __up    request self
Receive..08-45> ls -l
__trc   = {exc_context, thread, framestack, frame, related, funcname, context, exc_stack, where, id, argvalues}
__up   = {path_comps, __trc, __up, __down, number, self, recv, query_args}
request = <__main__.GetHandler instance at 0x106760fc8>
self    = <__main__.Receive object at 0x1068cb090>
Receive..08-45> cd __trc
osh..__trc> ls
argvalues   context     exc_context exc_stack   frame       framestack  funcname
id          related     thread      where
osh..__trc> ls -l where
where =
'__bootstrap-->__bootstrap_inner-->run-->process_request_thread-->finish_request-->__init__-->handle-->handle_one_request-->do_GET-->respond'

1.6   edit and traceassert

The edit command is perhaps the most useful command in otrace. It allows you to modify (monkey patch) any function or method in the running program. In particular, it makes it easy to use the “oldest” debugging technique, viz., inserting print statements in the code, without having to modify the actual source code files. You can also insert calls to traceassert, which is the equivalent of setting conditional breakpoints in the code.

Now that we know the there is an exception occurring in the method respond, we pretend that we don’t know the exact cause, and will use traceassert to determine the cause. The traceassert function has the signature traceassert(condition, label="", action=""). As long as condition is true, traceassert simply returns. If condition is false, the call is logged and a trace context virtual directory is created.

We suspect that the exception is caused because the user entered a number that was too small. First, we switch off safe mode, which disallows code editing. We then use the edit command to modify the respond method in the running program to insert a call to traceassert. (Actually hello_trace.py already has a traceassert call that is commented out. We simply uncomment it, as well as the diagnostic print statement, via the edit command.):

osh..__trc> cd ~~g
globals> set safe_mode False
safe_mode = False
globals> set trace_active True
trace_active = True
globals> edit Receive.respond
Patched Receive.respond:

Note that we need to activate tracing explicitly by setting parameter trace_active to True to trace traceassert calls. (This step is not needed when the trace command is used, because tracing is automatically activated.) After the edit, the statement otrace.traceassert(number > 0.001, label="num_check") has been inserted into Receive.respond. In the browser, enter the number 2 and then the number 0.0005. The latter value triggers a false condition on the traceassert. We switch to the assert trace context directory /osh/recent/asserts/Receive.respond/as-num_check/04-57-54, which allows us to examine the local variables when the assertion failed:

globals> submit(2)
rootW path=/?number=2
  <span>The reciprocal of 2.0 is 0.5</span>
globals> submit(0.0005)
rootW path=/?number=0.0005
Client address ('127.0.0.1', 62008)
Receive.respond:as-num_check:05-08-51
  <span>The reciprocal of 0.0005 is 2000.0</span>
globals> ls ~~
/osh/recent/asserts/Receive.respond/as-num_check/05-08-51
globals> cd ~~
Receive..08-51> ls
__up  __trc   request self
Receive..08-51> self.value
0.0005
Receive..08-51> request.headers
Accept-Encoding: identity
Host: 127.0.0.1:8888
Connection: close
User-Agent: Python-urllib/2.7

The default action when the traceassert condition is false is to create the trace context directory. The action argument to traceassert can be used set a breakpoint when the assertion fails (as explained later). For efficiency, the trace context for traceassert does not save the backtrace stack local variables or source code information by default. To enable backtracing of stack and source code, set assert_context to a non-zero value.

1.7   unpatch and untrace

After debugging is complete, the unpatch command can be used to restore the original code for Receive.respond. The untrace command can be used to switch off tracing:

globals> cd /osh/patches
patches> ls
Receive.respond
patches> unpatch Receive.respond
Unpatching Receive.respond
patches> cd ~~g
patches> trace
Receive.respond
globals> untrace Receive.respond
untraced Receive.respond
globals>

1.8   Object tag tracing

One of the allowed actions in the trace -a <action> -c <condition> ... command is tag. The tag action adds a special attribute to the self object if the trace condition is met at the time a function returns. The tag attribute is just a string, usually the object’s id, but can also be the current time or some other string. The presence of tagged arguments can be specified as a trace condition for subsequent tracing of a function, using the -c tagged[<argname>] option. The commands tag and untag can be used to directly add/remove the tag attribute.

In the example below, the method Receive.__init__ will tag the self object if self.value equals 1 when the method returns. Then, we trace Receive.respond if its argument named self is tagged. First, we submit the value 2, which does not trigger tagging. Next, we submit the value 1, which causes the self object to be tagged when Receive.__init__ returns, and then triggers a trace context for Receive.respond because one of its arguments is tagged:

globals> cd ~~g
globals> trace -a tag -c self.value==1 Receive.__init__
Tracing -a tag -c {'self.value==': 1L} Receive.__init__
globals> trace -c taggedself Receive.respond
Tracing -c taggedself Receive.respond
globals> submit(2)
rootW path=/?number=2
  <span>The reciprocal of 2.0 is 0.5</span>
globals> submit(1)
rootW path=/?number=1
Receive:tg-self.value==1;0x103231090:17-04-42
Receive.respond:tr-taggedself;tg-self.value==1;0x103231090:17-04-42.0 self=<__main__.Receive object at 0x103231090>, request=<__main__.GetHandler instance at 0x10322f950>
  <span>The reciprocal of 1.0 is 1.0</span>
globals> cd ~~
Receive..04-42.0> pwd
/osh/recent/traces/Receive.respond/tr-taggedself;tg-self.value==1;0x103231090/17-04-42.0
Receive..04-42.0> ls
__trc   request self

1.9   Preserving trace contexts

Only a limited number of trace contexts (controlled by set max_recent) are retained in memory. If there are too many contexts, the oldest contexts are deleted. The save command can be used to preserve trace contexts in memory. The set pickle_file command can be used to specify a pickle database file in which to save all trace contexts. This pickle database file can be opened at a later time using the unpickle command to read trace contexts into the /osh/pickled directory:

Receive..04-42.0> cd ~~g
globals> set pickle_file trace_test.db
pickle_file = trace_test.db
globals> trace Receive.respond
Tracing Receive.respond
globals> submit(0)
...
globals> unpickle trace_test.db
globals> cd /osh/pickled
pickled> ls
exceptions
pickled> cd exceptions/Receive.respond/ex-ZeroDivisionError/17-06-22.0/:
osh..:> ls
__trc   __up    request self
osh..:>

The special directory : is used to denote the content of the trace context.

1.10   Monitoring using the repeat command

The repeat command indefinitely repeats whatever command that follows it, erasing the screen each time before displaying the output. The default repeat interval is 0.2 seconds, and that can be changed via the set repeat_interval command. Any user input, or a trace event will end the repeat cycle. Here’s an example of using repeat to monitor requests processed by the demo web server:

> repeat ls -l Request_stats/*

1.11   Breakpoints

Breakpoints can be set using the -a break option for the trace command, or the action="break" argument to traceassert. In a multi-threaded program, there may be more than one breakpoint active at a time. To resume execution, issue the resume command when the current directory is within the trace context of the breakpoint to be resumed, or specify the trace id to be resumed as the argument.

NOTE: The trace command cannot set breakpoints at specific lines. Use the edit command to insert traceassert calls to break at specific locations within a function or method.

Other possible actions for trace and traceassert include pdb or ipdb, which launch the respective debuggers at the breakpoint. The continue command of these debuggers should be used to return control of the terminal to otrace.

Last modified: 2013-01-11