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Python has a small set of extremely useful built-in functions. All other functions are partitioned off into modules. This was actually a conscious design decision, to keep the core language from getting bloated like other scripting languages (cough cough, Visual Basic).
The type function returns the datatype of any arbitrary object. The possible types are listed in the types module. This is useful for helper functions which can handle several types of data.
>>> type(1)<type 'int'> >>> li = [] >>> type(li)
<type 'list'> >>> import odbchelper >>> type(odbchelper)
<type 'module'> >>> import types
>>> type(odbchelper) == types.ModuleType 1
The str coerces data into a string. Every datatype can be coerced into a string.
>>> str(1)'1' >>> horsemen = ['war', 'pestilence', 'famine'] >>> horsemen.append('Powerbuilder') >>> str(horsemen)
"['war', 'pestilence', 'famine', 'Powerbuilder']" >>> str(odbchelper)
"<module 'odbchelper' from 'c:\\docbook\\dip\\py\\odbchelper.py'>" >>> str(None)
'None'
At the heart of our help function is the powerful dir function. dir returns a list of the attributes and methods of any object: modules, functions, strings, lists, dictionaries… pretty much anything.
>>> li = [] >>> dir(li)['append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort'] >>> d = {} >>> dir(d)
['clear', 'copy', 'get', 'has_key', 'items', 'keys', 'setdefault', 'update', 'values'] >>> import odbchelper >>> dir(odbchelper)
['__builtins__', '__doc__', '__file__', '__name__', 'buildConnectionString']
![]() | li is a list, so dir(li) returns a list of all the methods of a list. Note that the returned list contains the names of the methods as strings, not the methods themselves. |
![]() | d is a dictionary, so dir(d) returns a list of the names of dictionary methods. At least one of these, keys, should look familiar. |
![]() | This is where it really gets interesting. odbchelper is a module, so dir(odbchelper) returns a list of all kinds of stuff defined in the module, including built-in attributes, like __name__, and whatever other attributes and methods you define. In this case, odbchelper has only one user-defined method, the buildConnectionString function we studied in Chapter 1. |
type, str, dir, and all the rest of Python's built-in functions are grouped into a special module called __builtins__. (That's two underscores before and after.) If it helps, you can think of Python automatically executing from __builtins__ import * on startup, which imports all the “built-in” functions into the namespace so you can use them directly.
The advantage of thinking like this is that you can access all the built-in functions and attributes as a group by getting information about the __builtins__ module. And guess what, we have a function for that; it's called help. Try it yourself and skim through the list now; we'll dive into some of the more important functions later. (Some of the built-in error classes, like AttributeError, should already look familiar.)
Example 2.11. Built-in attributes and functions
>>> from apihelper import help >>> help(__builtins__, 20) ArithmeticError Base class for arithmetic errors. AssertionError Assertion failed. AttributeError Attribute not found. EOFError Read beyond end of file. EnvironmentError Base class for I/O related errors. Exception Common base class for all exceptions. FloatingPointError Floating point operation failed. IOError I/O operation failed. […snip…]
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Python comes with excellent reference manuals, which you should peruse thoroughly to learn all the modules Python has to offer. But whereas in most languages you would find yourself referring back to the manuals (or man pages, or, God help you, MSDN) to remind yourself how to use these modules, Python is largely self-documenting. |
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