Financial Modelling in Python

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Appendix A: Python 203

Unlike C++ with its notion of private, protected and public access mechanisms,

everything in a class is publicly accessible and due to Python’s dynamic nature the definition

of a class can even be changed during execution of the script(!):

>>> def salary(mgr):

... return mgr.salary

...

>>> print salary(m)

20000

>>> manager.earns = salary

>>> print m.earns()

20000

>>>

Programming with classes is ubiquitous in Python and this section just covers the basics.

The newcomer to Python is recommended to take the time to become more familiar with them

early on.

A.2.6 Modules and Packages

A module is a lexical unit of Python code stored in a file on disk. Let us suppose the existence

of a file ‘complex.py’ with contents something like the following:

class complex(object):

def

init (self, re, im):

self.re, self.im = (re, im)

def real

part(self):

return self.re

def imag

part(self):

return self.im

def

str (self):

return "%f + %fi" % (self.re, self.im)

def

add (self, other):

return complex(self.re + other.re, self.im + other.im)

# ...

def conjugate(x):

return complex(x.real

part(), -x.imag part())

# ...

We can use the Python import directive to bring all of the type declarations and function

definitions defined in ‘complex.py’ into the current scope like this:

>>> from complex import *

>>> i = complex(0, 1)

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