27.1 Exploring on Your Own
Before digging into specific tasks, we should say a brief word about
self-exploration. We have not been exhaustive in coverage of object
attributes or module contents in order to focus on the most important
aspects of the objects under discussion. If you're
curious about what we've left out, you can look it
up in the Library Reference, or you can poke around in the Python
interactive interpreter, as shown in this section.
The dir built-in function returns a list of all
of the attributes of an object, and, along with the
type built-in, provides a great way to learn about
the objects you're manipulating. For example:
>>> dir([ ]) # What are the attributes of lists?
['__add__', '__class__', '__contains__', '__delattr__', '__delitem__',
'__delslice__', '__doc__', '__eq__', '__ge__', '__getattribute__', '__getitem__',
'__getslice__', '__gt__', '__hash__', '__iadd__', '__imul__', '__init__',
'__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__', '__reduce__',
'__repr__', '__rmul__', '__setattr__', '__setitem__', '__setslice__', '__str__',
'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort']
What this tells you is that the empty list object has a few methods:
append, count,
extend, index,
insert, pop,
remove, reverse,
sort, and a lot of "special
methods" that start with an underscore
(_) or two (__). These are
used under the hood by Python when performing operations like
+. Since these special methods are not needed very
often, we'll write a simple utility function that
will not display them:
>>> def mydir(obj):
... orig_dir = dir(obj)
... return [item for item in orig_dir if not item.startswith('_')]
...
>>>
Using this new function on the same empty list yields:
>>> mydir([ ]) # What are the attributes of lists?
['append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort']
You can then explore any Python object:
>>> mydir(( )) # What are the attributes of tuples?
[ ] # Note: no "normal" attributes
>>> import sys # What are the attributes of files?
>>> mydir(sys.stdin) # What are the attributes of files?
['close', 'closed', 'fileno', 'flush', 'isatty', 'mode', 'name', 'read', 'readinto',
'readline', 'readlines', 'seek', 'softspace', 'tell', 'truncate', 'write',
'writelines', 'xreadlines']
>>> mydir(sys) # Modules are objects too.
['argv', 'builtin_module_names', 'byteorder', 'copyright', 'displayhook',
'dllhandle', 'exc_info', 'exc_type', 'excepthook', 'exec_prefix', 'executable',
'exit', 'getdefaultencoding', 'getrecursionlimit', 'getrefcount', 'hexversion',
'last_traceback', 'last_type', 'last_value', 'maxint', 'maxunicode', 'modules',
'path', 'platform', 'prefix', 'ps1', 'ps2', 'setcheckinterval', 'setprofile',
'setrecursionlimit', 'settrace', 'stderr', 'stdin', 'stdout', 'version',
'version_info', 'warnoptions', 'winver']
>>> type(sys.version) # What kind of thing is 'version'?
<type 'string'>
>>> print repr(sys.version) # What is the value of this string?
'2.3a1 (#38, Dec 31 2002, 17:53:59) [MSC v.1200 32 bit (Intel)]'
Recent versions of Python also contain a built-in that is very helpul
to beginners, named (appropriately enough) help:
>>> help(sys)
Help on built-in module sys:
NAME
sys
FILE
(built-in)
DESCRIPTION
This module provides access to some objects used or maintained by the
interpreter and to functions that interact strongly with the
interpreter.
Dynamic objects:
argv--command line arguments; argv[0] is the script pathname if known
path--module search path; path[0] is the script directory, else ''
modules--dictionary of loaded modules
displayhook--called to show results in an interactive session
excepthook--called to handle any uncaught exception other than
SystemExit
To customize printing in an interactive session or to install a
custom top-level exception handler, assign other functions to replace
these.
...
There is quite a lot to the online help system. We recommend that you
start it first in its "modal"
state, just by typing help( ). From then on,
any string you type will yield its documentation. Type
quit to leave the help mode.
>>> help( )
Welcome to Python 2.2! This is the online help utility
...
help> socket
Help on module socket:
NAME
socket
FILE
c:\python22\lib\socket.py
DESCRIPTION
This module provides socket operations and some related functions.
On Unix, it supports IP (Internet Protocol) and Unix domain sockets.
On other systems, it only supports IP. Functions specific for a
socket are available as methods of the socket object.
Functions:
socket( ) -- create a new socket object
fromfd( ) -- create a socket object from an open file descriptor [*]
gethostname( ) -- return the current hostname
gethostbyname( ) -- map a hostname to its IP number
gethostbyaddr( ) -- map an IP number or hostname to DNS info
getservbyname( ) -- map a service name and a protocol name to a port
...
help> keywords
Here is a list of the Python keywords. Enter any keyword to get more help.
and elif global or
assert else if pass
break except import print
class exec in raise
continue finally is return
def for lambda try
del from not while
help> topics
Here is a list of available topics. Enter any topic name to get more help.
ASSERTION DEBUGGING LITERALS SEQUENCEMETHODS1
ASSIGNMENT DELETION LOOPING SEQUENCEMETHODS2
ATTRIBUTEMETHODS DICTIONARIES MAPPINGMETHODS SEQUENCES
ATTRIBUTES DICTIONARYLITERALS MAPPINGS SHIFTING
AUGMENTEDASSIGNMENT ELLIPSIS METHODS SLICINGS
BACKQUOTES EXCEPTIONS MODULES SPECIALATTRIBUTES
BASICMETHODS EXECUTION NAMESPACES SPECIALIDENTIFIERS
BINARY EXPRESSIONS NONE SPECIALMETHODS
BITWISE FILES NUMBERMETHODS STRINGMETHODS
BOOLEAN FLOAT NUMBERS STRINGS
CALLABLEMETHODS FORMATTING OBJECTS SUBSCRIPTS
CALLS FRAMEOBJECTS OPERATORS TRACEBACKS
CLASSES FRAMES PACKAGES TRUTHVALUE
CODEOBJECTS FUNCTIONS POWER TUPLELITERALS
COERCIONS IDENTIFIERS PRECEDENCE TUPLES
COMPARISON IMPORTING PRINTING TYPEOBJECTS
COMPLEX INTEGER PRIVATENAMES TYPES
CONDITIONAL LISTLITERALS RETURNING UNARY
CONVERSIONS LISTS SCOPING UNICODE
help> TYPES
3.2 The standard type hierarchy
Below is a list of the types that are built into Python. Extension
modules written in C can define additional types. Future versions of
Python may add types to the type hierarchy (e.g., rational numbers,
efficiently stored arrays of integers, etc.).
...
help> quit
>>>
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