This seems like a somewhat simple question that I'm having a lot of trouble finding an answer for, perhaps I haven't found the words programmers use to talk about this.
I am a relatively inexperienced programmer, and have run into some difficulties with troubleshooting an application I have made. The question I have could be general, because I haven't found an answer to this for any language, but for me I'm specifically using python and PyQt4:
Is there any way to view the behind-the-scenes calls made when I execute a method/call a command? I can use debugging software (in my case winpdb) to track which lines in my code are called, but, aside from traceback info given after an error, haven't figured out how to follow the program's steps "behind the scenes". This would be helpful for cases where there hasn't been a programming error from the compiler's point of view, but the behavior is unexpected because the programmer doesn't have a complete understanding of a module's inner workings.
To put it another way: I want to know about the code that I didn't write. I want to know what is triggered after a line of my code is called. If I call a method like len(), python calls its own methods that will eventually return an integer to me. I'm hoping there's a way we can see what python does in between my lines of code.
If this has been asked before, please let me know, and accept my apologies for repeating the question, but I haven't been able to find an answer to that question, or at least the best way of asking it. Thank you very much for your help.
I'm not sure what your requirements are, but using cProfile with Gprof2Dot will generate a png of your code's call graphs and the amount of time spent in each function/method.
Just:
grab gprof2dot.py
Use the given sample code
I've looked at most of the IDE's out there. I've set up vim to use autocompletion and I'm using it right now. However, I can't seem to get it to work like Visual Studio with .NET. Autocompletion seems to work only in certain cases and it only shows methods and not what parameters they take. It's pretty much unusable to me.
What I'm after is a pop-up that will show me all methods available and the parameters they take. Pretty much the feel of VS2010 when you're programming .NET.
You won't get the kind of autocompletion in a dynamic language like Python that you get in more explicitly typed languages. Consider:
def MyFunction(MyArg):
MyArg.
When you type the "." in MyArg., you expect the editor to provide a list of methods with arguments. That can't happen in Python because the editor has absolutely no way of knowing what type (or types) MyArg could possibly be. Even the Python compiler doesn't have that information when it's compiling the code. That's why, if you put MyArg.SomeNonExistentFunction() you won't get any kind of error message until runtime.
If you wrote something like:
def MyFunction:
MyObject = MyClass(SomeArg)
MyObject.
then a smart enough editor can supply a list of methods available after that final ".".
You'll find that those editors that are supplying autocomplete "sometimes" are doing so in cases similar to my second example, and not doing so in cases similar to the first. With Python, that's as good as you can get.
I've been using Eclipse with the PyDev extension for some time now. The auto-completion there is really quite impressive, I highly recommend it.
Gedit has a developer plugin which tries to do some syntax completion. For reasons already mentioned, it doesn't work very well. I found it more annoying than helpful and disabled it after a few weeks trial.
ipython's new Qt console has tab completion and you can have some tooltip sort of popups with syntax help and docstrings. See screenshot below for example..
But as most people have already pointed out, this kind of thing you are asking for is really more appropriate for less dynamic languages.
I've been using bpython for a while now for all of my Python interpreting needs. It's delightful, particularly when you're using unfamiliar new libraries, or libraries with a multitude of functions. In any case, it's nice to have a bpython interpreter running alongside what I'm doing, but it'd be even better if I had both the autocomplete-like feature, and the parameter description in the manner that bpython does while I'm editing code in Emacs. Am I completely crazy? Does anyone have an idea on how to do this?
Thanks,
Bradley Powers
You're not completely crazy.
python-mode can integrate with eldoc-mode to display the arg spec of the function you're calling at point. Just do M-x eldoc-mode while you're in a python file to turn it on and it should start working. It talks to an inferior python buffer to inspect the functions directly, so it should always be decently accurate. You can turn it on automatically for all new python-mode buffers with (add-hook 'python-mode-hook '(lambda () (eldoc-mode 1)) t) in your emacs startup file. Now, at this point I have to say that I don't do any regular python programming, and that when I tried it just now it didn't work. I spent a few minutes poking around in the source code and everything seems to be in place, but the code that it runs in the inferior process is just returning an empty string. Perhaps it's just my setup, or perhaps I'm reading the wrong source files; it's hard to say.
Emacs provides several different types of expansion/autocompletion. By default you have access to dabbrev-expand by hitting M-/. This is a fairly simple form of completion; it's just meant to work on any old file you happen to edit. More sophisticated is hippie-expand, but even that doesn't do anything python-specific. The documentation says that it can integrate with hippie-expand for exact completions, but this might be a lie; I couldn't figure out how it works. A little poking around shows several related solutions for this, all of which seem to rely on pymacs. If I were going to do a lot of python programming and didn't already have a fairly complicated emacs set up, I'd probably start by installing emacs-for-python. It looks to be a pretty complete setup, and even claims to have live warning/error detection.
In the spirit of helping others to help themselves, I'd like to point out how I came upon all of this information. My first step was to open a file in python-mode. I didn't actually have any python code available, so I just went to my scratch buffer and made it a python buffer (M-x python-mode). Then I asked for help about this strange new mode (C-h m) to see what it could do. It's author has kindly put a brief summary of what the mode can do which mentions eldoc-mode, Imenu, outline-mode, hippie-expand, rlcompleter, abbrev tables, and a bunch of other things. From there I started looking at the source code. For instance, to integrate with eldoc-mode, it defines a function called python-eldoc-function and gives that to the eldoc module for use in python buffers. Reading that code shows me how it interacts with the inferior buffer, etc.
I hope some of this helps.
I often use ipython (or the regular python shell) to test python code snippets while coding, and it's been very useful. One shortcoming of this, though, is that if I want to test a multi-line segment of code, or want to write multiple lines of code before running, it isn't very convenient to have to do it "line by line". And even going back to change some of the lines is cumbersome because you have to re-type all the code that comes after it.
I'm playing with Groovy right now and I find that it has an excellent solution to this problem: the Groovy Console. You just write all the code you want, it's just like a regular editor; and then you hit run Ctrl+R (Cmd+R actually since I'm on a Mac) and it runs everything at once. If you want to change something (e.g. if there are errors), then that's easy too -- just change it and Ctrl+R again.
Is there an equivalent of this available for python? Or do you have any recommendations on a way to achieve similar behavior? I could just create a new file, save it, and then python <filename>.py from the shell. But that's just too many steps and would be cumbersome. Eclipse may be an option, but it's too heavyweight. I'm really looking for something lightweight that I can just spin up when I want to test something and then get rid of it just as quickly.
I'd be interested to hear any ideas/suggestions!
Thanks
You might give DreamPie a try. As far as I can tell from a quick read of the groovyConsole page you linked to, DreamPie features a similar input area/output area division (they call it "code box" and "history box"). The code you execute is by default cleared from the code box - which groovyConsole apparently doesn't do - but you can easily retrieve it (Ctrl+Up), or change a preference setting to "Leave code in the code box after execution".
Have you tried using IDLE, the standard Python IDE? You'd have to save the code as <filename>.py within IDLE, but after that you can run it using F5.
The Python docs link to this intro to IDLE, which may be helpful even if it's a little outdated.
I am using emacs and its python-mode.
C-c C-c: evals the current buffer
but you can also eval region (ie selection), functions etc ...
You can even make python-mode use ipython (like I do).
See http://ipython.scipy.org/dist/ipython.el . It works nicely
Did you try PyCrust? It has excellent multi-line editing, copy/paste support.
PyCrust can be found in wxPython Docs and Demos.
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I'm getting quite a few errors in my code. Consequently, I would like to be able to minimize them at the outset or see as many errors as possible before code execution. Is this possible and if so, how can I do this?
If you're having problems with syntax, you could try an editor with syntax highlighting. Until you get the feel for a language, simple errors won't just pop out at you.
The simplest form of debugging is just to insert some print statements. A more advanced (and extensible) way to do this would be to use the logging module from the std lib.
The interactive interpreter is a wonderful tool for working with python code, and IPython is a great improvement over the built-in REPL (Read Eval Print Loop).
If you actually want to step through your code, the python debugger is called pdb, which can be called from the command line, or embedded in your code.
If you're used to a fully integrated IDE, I would recommend using Eclipse with pydev, and PyCharm has a great commercial offering, with autocomplete, quick access to docs, and numerous shortcuts, among many other interesting features.
Here is some techniques to facilitate debugging in Python:
use interactive shell e.g., ipython. Python is a dynamic language you can explore your code as you type. The shell is running in the second window in my editor at all times.
copy-paste from the shell into docstrings a code that illustrates a dominant usage and corner cases of the function (class, module). doctest.testmod() placed in a if __name__=="__main__" section allows to test all docstrings in the module. doctest can be easily integrated with unittest.
use assert for stuff that can never happen.
print() can solve many debugging problems; logging module is suitable for long-living python processes.
write tests (not necessarily before your code), run them often (automatically or with one keystroke at most); nose provides extended test discovery and running features for unittest.
run pylint occasionally.
At this point there is a little use for a formal python debugger.
Winpdb is an external multi-platform GPL'ed GUI python debugger if you need one.
All the really cool stuff is easily demonstrated in the interactive interpreter. I think this might be the "gold standard" for good design:
Can you exercise your class interactively?
If you can do stuff interactively, then you can write unittests and doctests with confidence that it's testable, simple, reliable.
And, more important, you can explore it interactively. There's nothing better than the instant gratification that comes from typing code and seeing exactly what happens.
The compiled language habits (write a bunch of stuff, debug a bunch of stuff, test a bunch of stuff) can be left behind. Instead, you can write a little bit of stuff, explore it, write a formal test and then attach your little bit of stuff to your growing final project.
You still do overall design. But you don't squander time writing code that may or may not work. In Python you write code that works. If you're not sure, you play with it interactively until you're sure. Then you write code that works.
I am new to python, and have been trying several different debuggers. Here are the options I've come across so far:
Eclipse with Pydev - If you're already using eclipse, this is probably the way to go. The debugger works well, and is pretty featureful.
Komodo IDE - A light-weight python IDE. Basically a text editor + a debugger. It was really easy to figure out and be productive with immediately.
WinPDB - Trying this one next. Looks very featureful, and I get to use whichever editor I choose.
PDB - Haven't tried yet since I've read about how WinPDB is a better alternative.
Ipython with %run command - I've used IPython, but not as a debugger like this. I need to try this out. (Thanks for the tip, EOL)
Eric IDE - Also on the list to try.
Old-school print, assert statements - Simple, useful, but not a full solution.
Memory debugging - To debug memory problems, I've come across a few tools:
objgraph - Will generate png's of reference graphs. Neat. There's other functionality as well, like: import objgraph;print(objgraph.show_most_common_types(limit=10)) will print the top 10 most common types in memory.
gc module - Interact directly with the garbage collector.
heapy - Heap analyzer. For example: from guppy import hpy; hp = hpy(); print(hp.heap()) will print the most common types, their memory usage, etc.
This is a very incomplete list, but hopefully it's a good start.
Python provides a debugger which allows you to step through your code, inspect variables and manipulate them. Refer to http://pythonconquerstheuniverse.wordpress.com/category/python-debugger/ which can take you over the steps...
Also check the python standard library reference for pdb
http://www.python.org/doc/2.5.2/lib/module-pdb.html
Test early and test often.
This doesn't necessarily mean to jump into the test driven design pool head first (though that's not such a bad idea). It just means, test your objects and methods as soon as you have something that works. Don't wait until you have a huge pile of code before doing testing.
Invest some time in learning a testing framework. If it's trivial for you to type in a test case you'll more likely do it. If you don't have any sort of framework testing can be a pain so you'll avoid it. So, establish some good habits early on and you'll have fewer problems down the road.
More often than not, I just use a bunch of print statements.
page = grabpage(url)
print "Page content:", page
print "page type():", type(page)
It's sometimes useful to do something like:
debug = True
if debug: print "page content", page
..with this you can quickly disable all your debugging print statements by changing the debug variable to False.
While print-debugging will get you very far in most cases, sometimes it's difficult to debug things like loops, or a series of if/else/try/except/etc. For this a debugger that allows stepping though your code, and setting break-points is useful.
pdb is included with Python. Here is a good simple tutorial on it. You can even do things like changing variables during run-time (which the tutorial covers). A longer tutorial can be found here
There is a very nice GUI equivalent pdb - Winpdb
Basically you run winpdb myscript --arg 4 -b 4 then it loads the command in a terminal, shows you your code on the left, with the current, a list of local/global variables (and their values) and the current call-stack.
Then you can step though the code by clicking the Step (or F6). F5 runs the code. If you click next to the line numbers, it sets a break point, where the code will automatically step (when you press run).
I find it far easier to use, and it has lots of addition enhancements (like remote debugging, so you can run the backend portion (rpdb2) in the to-be-debugged application, and connect Winpdb to it (encrypted). It also has support for threading and other useful things not in PDB. You have access to a pdb-like console too.
I set up Python to automatically start the debugger when there's an uncaught exception, using this trick. That way, you can easily examine the state of the program without too much logging code. (Plus, to send me a Growl notification.)
Oh, and this way you can just create a break point in the code by adding
if answer == 42:
1/0
Using assert statement liberally.
Identifing errors before execution is the domain of static checking/analysis. I've had good luck using PyChecker for basic static checking of Python code.
The pycheesecake site has a very good summary of testing tools for Python.
The PyDev plugin for eclipse is my tool of choice. It recognizes simple syntax mistakes and indentation errors and underlines the error with a red line. It has a powerful debugger and even has a plugin called PyLint which warns you about dangerous code.
Edit:
It also has a user friendly stack trace on runtime errors, partial auto complete and some other nifty features.
Edit again:
I didn't notice that pydev was mentioned in the top post. I hope I brought something else to the table.
python -m pdb yourcode.py should do it.
Alternatively you can "import pdb" in your code and use pdb.set_trace() to set break points.
Refer the manual for more info: http://www.python.org/doc/2.5.2/lib/module-pdb.html
There is very nice GUI debugger for Python called Winpdb. Try it out.Built on wxWidgets library and multiplatform.
For a most integrated experience you can use full blown IDE like PyCharm:
http://blog.jetbrains.com/pycharm/files/2011/05/template-debug2.png
Eric4 IDE also has a great built-in debugger.
The IPython Python shell has a %pdb command that automatically invokes the debugger in case of problem. You can then inspect variables, step through the code, etc.
You can run your programs from IPython with the %run command.
More generally, as as been noted in some other answers, a good Python shell is your friend. And IPython is your best friend. :)