taxes: explanatory material, recursive implementation of split_by_bracket

Author: drocco007

2013-04/intro_taxes/README.rst

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+
+==================
+The Tax Man Cometh
+==================
+
+---------------------------
+Or: The Other Inevitability
+---------------------------
+
+
+:Author: Daniel J. Rocco, Ph.D.
+
+
+Tax season gave me an excuse to look at the ``csv`` module to show how one 
+could read in CSV data, manipulate it, and write out a CSV file using Python.
+
+Topics covered:
+
+*   reading in a CSV file
+*   creating a lightweight model of a tax bracket using ``namedtuple``
+*   using ``map`` to call a function on each element of a list
+*   massaging the string data from a CSV into a more useful numeric format with
+    Python's ``Decimal`` class
+*   problem analysis: breaking down the "compute tax" problem into a set of 
+    prerequisites and a list of smaller steps
+*   review of some of Python's list mechanisms including comprehensions and the 
+    ``sum`` function
+*   writing out a CSV file
+
+
+Organization
+============
+
+``01_csv2.py``
+    read in a CSV file and print out its rows
+    
+``01b_csv2.py``
+    read in a CSV file and convert its rows to a list
+
+``02_semantically_enhanced.py``
+    create a lightweight model of a tax bracket using ``namedtuple``; use the 
+    ``Bracket._make`` function to convert the rows of the CSV file to Bracket 
+    objects
+    
+``brackets.py``
+    further process the input data by converting strings to ``Decimal`` objects 
+    more suited for mathematical calculations
+
+``03_split_by_bracket.txt``
+    sketches out the design of our tax calculator, which includes the 
+    prerequisite information needed to perform the calculation and the steps 
+    involved in the calculation itself
+        
+``split_by_bracket.py``
+    implementation of 03 step 1, including some sample calculations
+        
+``tax.py``
+    implementation of the tax calculator, including some sample calculations
+        
+``tax_table.py``
+    uses ``tax.py`` to write out a tax table
+        
+``simple_brackets.csv``
+    the example brackets, which present a simplified progressive tax model.  
+    Changing the values here (e.g. using tax schedule data available on the 
+    web) will update the brackets used by the examples above
+
+
+Running the Examples
+--------------------
+
+I've added support code to the example Python programs above to allow them to 
+be run standalone.  All you will need is a working Python installation; for 
+example::
+
+    $ python tax.py
+    single $5000: 500.0
+    single $11000: 1200.0
+    single $24000: 4200.0
+    
+    married $7000: 700.0
+    married $15000: 1500.0
+    married $19000: 2300.0
+    married $31000: 4800.0
+
+
+Recursive implementation of ``split_by_bracket``
+------------------------------------------------
+
+Due to popular demand I have added a recursive implementation of 
+``split_by_bracket`` in ``split_by_bracket_recursive.py`` along with some 
+example runs.
+
+
+Resources
+=========
+
+*   `The Python Tutorial <http://docs.python.org/2/tutorial/>`_
+*   `The csv module <http://docs.python.org/2/library/csv.html>`_ 
+*   `The collections module, home of namedtuple <http://docs.python.org/2/library/collections.html>`_ 
+*   `Python's Decimal <http://docs.python.org/2/library/decimal.html>`_ 
+*   `Tax Bracket Article <http://en.wikipedia.org/wiki/Tax_bracket>`_; source of the simple brackets used in the demo

2013-04/intro_taxes/split_by_bracket_recursive.py

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+from brackets import brackets
+
+
+def split_by_bracket(earnings, filing_status, _i=0, _already_taxed=0):
+    bracket = brackets[_i]
+    bracket_amount = getattr(bracket, filing_status)
+
+    if not bracket_amount or earnings <= bracket_amount:
+        return [(earnings - _already_taxed, bracket.tax_rate)]
+    else:
+        return [(bracket_amount - _already_taxed, bracket.tax_rate)] + \
+                split_by_bracket(earnings, filing_status, 
+                    _i=_i + 1, _already_taxed=bracket_amount)
+
+
+if __name__ == '__main__':
+    for amount in (5000, 11000, 24000):
+        print 'single ${}:'.format(amount), \
+              split_by_bracket(amount, 'single')
+
+    print
+
+    for amount in (7000, 15000, 19000, 31000):
+        print 'married ${}:'.format(amount), \
+              split_by_bracket(amount, 'married_joint')