Python Code Snippets

Here I will keep some code that I could add to any of my projects as some utility module.

Iteration tools

Besides the itertools module itself, there is a big collection of recipes right in the official Python documentation. But I still wish to introduce some utility functions here.

import operator

def split_by_condition(condition, iterable):
    """split_by_condition(function or None, sequence) -> \
pair of lists, tuples, or strings
    
    Split sequence into two subsequences by condition and return them.
    """
    if condition is None:
        not_condition = operator.not_
    else:
        not_condition = lambda item: not condition(item)
    t = filter(condition, iterable)
    f = filter(not_condition, iterable)
    return t, f

def filter_element(element, iterable):
    """filter_element(object, iterable) -> list, tuple, or string

    Delete all occurrences of object in iterable and return new one."""
    # this is surely not a big deal, of course.
    # but take a look at this incredibly clear and simple solution:
    return filter(element.__ne__, iterable)
    # well, it actually doesn't work in 100% of situations because
    # sometimes `__ne__` raises NotImplementedError.

Math

from math import copysign, pi, atan2
from numbers import Number
from operator import mul

# sign function. I wonder why there aren't any in Python.
def sign(x):
    """sign(x)

    Return integer 1 or -1 depending on the sign of x.
    """
    return int(copysign(1, x))

# `prod` function, brother of `sum`.
def prod(iterable, start=1):
    """prod(sequence[, start]) -> value

    Returns the product of a sequence of elements, beginning with
    the value of parameter 'start' (which defaults to 1). When the
    sequence is empty, returns start."""
    return reduce(mul, iterable, start)
    # WARNING: `reduce` is in `itertools` module in Python 3

# function finding cardinal direction name.
def cardinal_direction(dx, dy):
    """cardinal_direction(dx, dy)

    Returns initials of cardinal direction pointed by (dx, dy).
    
    Usage examples:
    >>> cardinal_direction(1, 1)
    'NE'
    >>> cardinal_direction(-1, 1)
    'NW'
    """
    angle = atan2(y, x)
    angle_idx = int(4. * angle / pi + 8.5) % 8
    return ['E','NE','N','NW','W','SW','S','SE'][angle_idx]

# now i'm going to introduce the shortest vector math class possible.
# if you wish to use some vector math but don't want to depend on NumPy -
# this is the solution.
from numbers import Number
import operator

# some accessory decorators first.
def _vectorize(op, name=None, doc=None):
    def fn(self, other):
        if isinstance(other, Number):
            return vector(op(item, other) for item in self)
        else:
            return vector(map(op, self, other))
    fn.__name__ = name or op.__name__
    fn.__doc__  = doc
    return fn

def _reverse(op):
    return lambda *args: op(*args[::-1])

class vector(tuple):
    """The simpliest vector class possible.
    Supports basic vector-vector and vector-number
    arithmetical operations.
    """
    __add__  = _vectorize(operator.add, '__add__')
    __sub__  = _vectorize(operator.sub, '__sub__')
    __mul__  = _vectorize(operator.mul, '__mul__')
    __div__  = _vectorize(operator.div, '__div__')
    __radd__ = _vectorize(_reverse(operator.add), '__radd__')
    __rsub__ = _vectorize(_reverse(operator.sub), '__rsub__')
    __rmul__ = _vectorize(_reverse(operator.mul), '__rmul__')
    __rdiv__ = _vectorize(_reverse(operator.div), '__rdiv__')
    # add any operations you wish here.
  
    def __abs__(self):
      """Computes euclidian length of a vector."""
      return sum(abs(item) ** 2 for item in self) ** 0.5
    
    def transpose(self): # yep, it works on matrices as well
        return vector(map(vector, zip(*self)))

Strings

import re # this is promising

def split_by_capitals(s):
    """"split_by_capitals(S) -> string

    Return a copy of the string S with all case changes padded with spaces.

    >>> split_by_capitals('HiThere')
    ... 'Hi There'"""
    words = re.findall('[A-Z][^A-Z]*', s)
    return ' '.join(words)

def decapitalize(s):
    """decapitalize(S) -> string

    Return a copy of the string S with its first character
    lowercase."""
    return s[:1].lower() + s[1:]

def untitle(s):
    """untitle(S) -> string

    Return an untitlecased version of S, i. e. words start with lowercase
    characters, all remaining cased characters keep their case."""
    words = s.split(' ')
    words = map(decapitalize, words)
    return ' '.join(words)

Miscellaneous

def superposition(f, g, *args):
    """superposition(callable1, callable2, ...) -> function
    
    Create a function implementing superposition of arguments."""
    def result(*args, **kwargs):
        return f(g(*args, **kwargs))
    result.__name__ = '%s * %s' % (f.__name__, g.__name__)
    if args:
        result = superposition(result, *args)
    return result