Intro to Python Introspection and Dynamic Programming

The Type Function

Python supports dynamic typing, meaning that any symbol in the namespace can at anytime point to any type of object. Obviously, this could cause problems, fortunately the built- in function type() can be used to prevent problems. Hence:

>>> var1 = 12
>>> typ = str(type(var1))
>>> if typ == "<type 'int'>":
...     var1 / 4
... else: print 'Not an Int'
3
>>> var1 = 'abc'
>>> typ = str(type(var1))
>>> if typ == "<type 'int'>":
...     var1/4
... else: print 'Not an Int'
Not an Int

Python even allows every member of a list or dictionary, even the keys, to be an object of any type. This can be very useful in organizing data and procedures in a program in a style of a database. A simple example:

#!/usr/bin/env python
class demo:
    def __init__(self,empname, rate, action):
        self.__empname = empname
        self.__rate    = rate
        self.__action  = action

    def getVals(self):
        return self.__dict__

def domain():
    demo1 = demo('john doe', 12, 'regpay')
    vals  = demo1.getVals()
    for k in vals.keys():
        print k, str(type(vals[k]))

if __name__ == '__main__':
    domain()

Dir and Help

Python has many built-in modules. For instance one of the most useful is sys.

>>> import sys

And we can find out quickly what methods and properties it contains:

>>> dir(sys)
['__displayhook__', '__doc__', '__excepthook__', '__name__', 
'__stderr__', '__stdin__', '__stdout__', 
'_current_frames', '_getframe', 'api_version', 'argv', 
'builtin_module_names', 'byteorder', 
'call_tracing', 'callstats', 'copyright', 'displayhook', 'exc_clear', 
............
'setrecursionlimit', 'settrace', 'stderr', 'stdin', 'stdout', 
'subversion', 'version', 'version_info', 'warnoptions']

Notice it contains something called stderr, which we can discover to be a file:

>>> type(sys.stderr)

And if we need to know more, the following will give a fairly good description of the object type file:

>>> help(sys.stderr)

Python Introspection
Python Introspection
Dynamic Programming