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The dir Function

The built-in dir function is an easy way to grab a list of all the attributes available inside an object (i.e., its methods and simpler data items). It can be called on any object that has attributes. For example, to find out what’s available in the standard library’s sys module, import it and pass it to dir (these results are from Python 3.0; they might vary slightly on 2.6):

>>> import sys
>>> dir(sys)
['__displayhook__', '__doc__', '__excepthook__', '__name__', '__package__',
'__stderr__', '__stdin__', '__stdout__', '_clear_type_cache', '_current_frames',
'_getframe', 'api_version', 'argv', 'builtin_module_names', 'byteorder',
'call_tracing', 'callstats', 'copyright', 'displayhook', 'dllhandle',
'dont_write_bytecode', 'exc_info', 'excepthook', 'exec_prefix', 'executable',
'exit', 'flags', 'float_info', 'getcheckinterval', 'getdefaultencoding',
...more names omitted...]

Only some of the many names are displayed here; run these statements on your machine to see the full list.

To find out what attributes are provided in built-in object types, run dir on a literal (or existing instance) of the desired type. For example, to see list and string attributes, you can pass empty objects:

>>> dir([])
['__add__', '__class__', '__contains__', ...more...
'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove',
'reverse', 'sort']

>>> dir('')
['__add__', '__class__', '__contains__', ...more...
'capitalize', 'center', 'count', 'encode', 'endswith', 'expandtabs',
'find', 'format', 'index', 'isalnum', 'isalpha', 'isdecimal',
'isdigit', 'isidentifier', 'islower', 'isnumeric', 'isprintable',
'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', '
maketrans', 'partition', 'replace', 'rfind', 'rindex', 'rjust',
...more names omitted...]

dir results for any built-in type include a set of attributes that are related to the implementation of that type (technically, operator overloading methods); they all begin and end with double underscores to make them distinct, and you can safely ignore them at this point in the book.

Incidentally, you can achieve the same effect by passing a type name to dir instead of a literal:

>>> dir(str) == dir('')           # Same result as prior example
True
>>> dir(list) == dir([])
True

This works because names like str and list that were once type converter functions are actually names of types in Python today; calling one of these invokes its constructor to generate an instance of that type. I’ll have more to say about constructors and operator overloading methods when we discuss classes in Part VI.

The dir function serves as a sort of memory-jogger—it provides a list of attribute names, but it does not tell you anything about what those names mean. For such extra information, we need to move on to the next documentation source.