Chapter 12 - Introspection¶
Whether you’re new to Python, been using it for a few years or you’re an expert, knowing how to use Python’s introspection capabilities can help your understanding of your code and that new package you just downloaded with the crappy documentation. Introspection is a fancy word that means to observe oneself and ponder one’s thoughts, senses, and desires. In Python world, introspection is actually kind of similar. Introspection in this case is to use Python to figure out Python. In this chapter, you can learn how to use Python to give yourself a clue about the code you’re working on or trying to learn. Some might even call it a form of debugging.
Here’s what we’re going to cover:
The Python Type¶
You may not know this, but Python may just be your type. Yes, Python can tell you what type of variable you have or what type is returned from a function. It’s a very handy little tool. Let’s look at a few examples to make it all clear:
>>> x = "test" >>> y = 7 >>> z = None >>> type(x) <class 'str'> >>> type(y) <class 'int'> >>> type(z) <class 'NoneType'>
As you can see, Python has a keyword called type that can tell you what is what. In my real-life experiences, I’ve used type to help me figure out what is going on when my database data is corrupt or not what I expect. I just add a couple lines and print out each row’s data along with its type. This has helped me a lot when I’ve been dumbfounded by some stupid code I wrote.
The Python Dir¶
What is dir? Is it something you say when someone says or does something stupid? Not in this context! No, here on Planet Python, the dir keyword is used to tell the programmer what attributes and methods there are in the passed in object. If you forget to pass in an object, dir will return a list of names in the current scope. As usual, this is easier to understand with a few examples.
>>> dir("test") ['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__getslice__', '__gt__', '__hash__', '__init__', '__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__', '__rmul__', '__setattr__', '__str__', 'capitalize', 'center', 'count', 'decode', 'encode', 'endswith', 'expandtabs', 'find', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'replace', 'rfind', 'rindex', 'rjust', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']
Since everything in Python is an object, we can pass a string to dir and find out what methods it has. Pretty neat, huh? Now let’s try it with an imported module:
>>> import sys >>> dir(sys) ['__displayhook__', '__doc__', '__egginsert', '__excepthook__', '__name__', '__plen', '__stderr__', '__stdin__', '__stdout__', '_getframe', 'api_version', 'argv', 'builtin_module_names', 'byteorder', 'call_tracing', 'callstats', 'copyright', 'displayhook', 'dllhandle', 'exc_clear', 'exc_info', 'exc_traceback', 'exc_type', 'exc_value', 'excepthook', 'exec_prefix', 'executable', 'exit', 'exitfunc', 'getcheckinterval', 'getdefaultencoding', 'getfilesystemencoding', 'getrecursionlimit', 'getrefcount', 'getwindowsversion', 'hexversion', 'maxint', 'maxunicode', 'meta_path', 'modules', 'path', 'path_hooks', 'path_importer_cache', 'platform', 'prefix', 'setcheckinterval', 'setprofile', 'setrecursionlimit', 'settrace', 'stderr', 'stdin', 'stdout', 'version', 'version_info', 'warnoptions', 'winver']
Now, that’s handy! If you haven’t figured it out yet, the dir function is extremely handy for those 3rd party packages that you have downloaded (or will soon download) that have little to no documentation. How do you find out about what methods are available in these cases? Well, dir will help you figure it out. Of course, sometimes the documentation is in the code itself, which brings us to the builtin help utility.
Python comes with a handy help utility. Just type “help()” (minus the quotes) into a Python shell and you’ll see the following directions (Python version may vary)
>>> help() Welcome to Python 2.6! This is the online help utility. If this is your first time using Python, you should definitely check out the tutorial on the Internet at http://www.python.org/doc/tut/. Enter the name of any module, keyword, or topic to get help on writing Python programs and using Python modules. To quit this help utility and return to the interpreter, just type "quit". To get a list of available modules, keywords, or topics, type "modules", "keywords", or "topics". Each module also comes with a one-line summary of what it does; to list the modules whose summaries contain a given word such as "spam", type "modules spam". help>
Note that you now have a “help>” prompt instead of the “>>>”. When you are in help mode, you can explore the various modules, keywords and topics found in Python. Also note that when typing the word “modules”, you will see a delay as Python searches its library folder to acquire a list. If you have installed a lot of 3rd party modules, this can take quite a while, so be prepared to go fix yourself a mocha while you wait. Once it’s done, just follow the directions and play around with it and I think you’ll get the gist.
Now you know how to take an unknown module and learn a lot about it by just using some of Python’s built-in functionality. You will find yourself using these commands over and over again to help you learn Python. As I mentioned earlier, you will find these tools especially helpful for the 3rd Party modules that don’t believe in documentation.