Friday, April 19, 2013

Python str with custom truth values

I've run into multiple instances where I get a string from some external service like a config file or a database.  There are values that I want to treat as true and others as false for the purposes of logic in my code.  For example let's say I have a status string with a few possible values, some false and some true.
  1. "Yes", "on", "true"
  2. "No", "off", "false"
I can map the values to True or False in Python.  Now I just need a string I can configure what it's truth values are.

class BooleanString(str):
    def __nonzero__(self):
        return self.lower() in ('on', 'yes', 'true') 

bool(BooleanString("YES")) # return True
bool(BooleanString("some other value")) # returns False

The thing is I don't want to hard code the truth values in the class definition.  I want to pass them in to the creation of the derived class.

def mkboolstr(truth):
    def __nonzero__(self):
        return self.lower() in truth
    return type('BooleanString', (str, ), dict(__nonzero__=__nonzero__))

BooleanString = mkboolstr(('on', 'yes', 'true'))

bool(BooleanString("TRUE")) # return True
bool(BooleanString("some other value")) # returns False

And now we have a function which creates a BooleanString class using a parameterized string as the comparable truth.


Python Expressions: Merging Dictionaries

It's common in Python programming to need to merge 2 or more dictionaries together. 

The first idiom is using the dict constructor.  This idiom has it's limitations, however it will always work fine as long as the keys are all strings. Trying this with non-string keys will fail in Python 3.2 and later, and also fails in alternate Python implementations. The idiom itself is frowned upon.

d1 = dict(a=1, b=2, c=3)
d2 = dict(c=4, d=5, e=6)

# merging 2 dicts with the dict constructor
merged_dict = dict(d1, **d2)

# merging n dicts with the dict constructor
merged_dict = reduce(lambda a, b: dict(a, **b), (d1, d1))

There are also other choices which will work with any type of key. Unfortunately they require a tad more code.

# merging n dicts with a generator comprehension
merged_dict = dict(i for iterator in (d1, d2) for i in iterator.iteritems())

# merging n dicts with dict comprehension
merged_dict = {k:v for d in (d1, d2) for k, v in d.iteritems()}


I have left out dict.update because it is only usable in a statement, not an expression. It also modifies a dictionary which may not be what I want to do. You can compare:

def fn1(d1, d2):
   d3 = d1.copy()
   return d3

# vs

return dict(d1, **d2)

# or

return {k:v for d in (d1, d2) for k, v in d.iteritems()}

I prefer the expressions.

Thursday, April 18, 2013

Python Expressions: Dict Slicing

As anyone who comes from Perl can tell you hash slicing is useful.  Python dicts do not natively support slicing.  One of the issues is slicing in Python seems limited to defined ranges, rather than an ad-hoc collection of values.

Take heart! There is an expression that has the same effect in Python as Perl's hash slicing:

from operator import itemgetter

d = dict(a=1, b=2, c=3, d=4)
h, i, j, k = itemgetter('a', 'c', 'a', 'd')(d)
# h, i, j, k => 1, 3, 1, 4

Alternate solutions are usually more complicated expressions.
d = dict(a=1, b=2, c=3, d=4)

# conceptually more complicated expressions
h, i, j, k = (d[i] for i in ('a', 'c', 'a', 'd'))
h, i, j, k = map(d.get, ('a', 'c', 'a', 'd'))

itemgetter provides a more simple expression for extracting multiple ad-hoc values from a dictionary.