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Dispatch on Value for Python

This Python 2.7/Python 3.x package provides the ability to dispatch on values (as opposed to dispatching on types) by pairing functions with patterns. It uses pattern matching to dispatch on complex, nested data structures containing lists, dictionaries and primitive types. You can use lambda to do expression matching and utilise wildcard parameters to ensure identical values can be matched (see any_a). It can alleviate complicated and difficult to read if ... elif ... elif ... chains and simplify the code.

Value patterns can be registered dynamically, allowing a great flexibility in determining which functions are called on which value patterns.

The home page is on github at:

Install using pip:

pip install dispatchonvalue

Unit tests can be run using:

python -m unittest discover

Any queries and comments are welcome. Send them to:


Very quick example

First register your dispatch methods, alongside the pattern they should match on:

import dispatchonvalue as dv

dispatch_on_value = dv.DispatchOnValue()

# Register your overloaded functions:
@dispatch_on_value.add([1, 2, 3])  # [1, 2, 3] is the pattern to match on
def _(a):
    assert a == [1, 2, 3]
    # return optional value
    return 3

@dispatch_on_value.add([4, 5, 6])  # [4, 5, 6] is the pattern to match on
def _(a):
    assert a == [4, 5, 6]
    # return optional value
    return 4

Then else where in your code, dispatch to the correct function based on the value of the parameter passed:

p = [4, 5, 6]
r = dispatch_on_value.dispatch(p)  # Will call second function above

If no pattern was matched, and hence no function dispatched, the DispatchFailed class will be raised:

  p = [7, 8, 9]
  r = dispatch_on_value.dispatch(p)
except dv.DispatchFailed:
  print 'could not dispatch!'


Multiple dispatch on value

The simplest use is to dispatch on fixed values. Here we dispatch to two different functions fn_1 and fn_2 depending upon the value of p:

@dispatch_on_value.add([1, 2, 3])
def fn_1(a):
    assert a == [1, 2, 3]
    # Do something

@dispatch_on_value.add([4, 5, 6])
def fn_2(a):
    assert a == [4, 5, 6]
    # Do something

p = [1, 2, 3]
dispatch_on_value.dispatch(p)  # This will call fn_1 and return True

p = [4, 5, 6]
dispatch_on_value.dispatch(p)  # This will call fn_2 and return True

p = [1, 2, 6]
dispatch_on_value.dispatch(p)  # This will not call anything and return False

Data structure patterns can be arbitrary nested

The patterns can be as complex and as nested as you like:

@dispatch_on_value.add({'one': 3, 'animals': ['frog', 'mouse', 34]})

Insert Lambda for wide expression of patterns

Use lambda's as part of the pattern matching:

@dispatch_on_value.add([1, 2, lambda x: 3 < x < 7, 'hello'])
def _(a):
    # Do something

dispatch_on_value.dispatch([1, 2, 4, 'hello'])  # This will match
dispatch_on_value.dispatch([1, 2, 2, 'hello'])  # This will not match

Another example:

@dispatch_on_value.add(['a', 2, lambda x: x == 'b' or x == 'c'])
def _(a):
    # Do something

dispatch_on_value.dispatch(['a', 2, 'c'])  # This will match
dispatch_on_value.dispatch(['a', 2, 's'])  # This will not match

Wildcard parameters

Use of wildcard tokens any_a, any_b, ... any_z can ensure values are identical. e.g.:

@dispatch_on_value.add([dv.any_a, 'b', 3, [3, 'd', dv.any_a]])
def _(a):
    # Do something

dispatch_on_value.dispatch(['c', 'b', 3, [3, 'd', 'c']])  # This will match
dispatch_on_value.dispatch(['f', 'b', 3, [3, 'd', 'f']])  # This will match
dispatch_on_value.dispatch(['c', 'b', 3, [3, 'd', 'f']])  # This will not match

Match everything in a list with single token

Use the all_same token to see if all the items in a list match, e.g.:

@dispatch_on_value.add(['a', dv.all_same(4)])
def _(a):
    # Do something

# This will match as the nested list contains all fours
dispatch_on_value.dispatch(['a', [4,4,4,4,4,4,4]])

You can combine them with the any_X token:

@dispatch_on_value.add(['a', dv.all_same(dv.any_a)])
 def _(a):
     # Do something

 # These will match as the nested list contains all the same values
 dispatch_on_value.dispatch(['a', [4,4,4,4,4,4,4]])
 dispatch_on_value.dispatch(['a', [5,5,5]])

 # This won't match
 dispatch_on_value.dispatch(['a', [1,2,3]])

These examples are simplistic but a more complex example might be:

@dispatch_on_value.add(dv.all_same({'age': 32}))
def _(a):
    # Do something

# This would match since all the items in the list have the same age
dispatch_on_value.dispatch([{'name': 'john', 'age': 32},
                            {'hair': 'brown', 'age': 32, 'car': 'lada'}])

# This wouldn't match since the ages are different
dispatch_on_value.dispatch([{'name': 'john', 'age': 32},
                            {'name': 'john', 'age': 9}])

Another example:

# Match on a list of dictionaries where the name is 'john' and the
# age is between 30 and 40
@dispatch_on_value.add(dv.all_same({'name': 'john',
                                    'age': lamba x: 30 < x < 40})
def _(a):
    # Do something

# This would match
dispatch_on_value.dispatch([{'name': 'john', 'age': 32},
                            {'name': 'john', 'age': 37}])

# This would not match
dispatch_on_value.dispatch([{'name': 'john', 'age': 32},
                            {'name': 'john', 'age': 45}])

No limit on parameters

Pass as many extra parameters as you want when dispatching:

@dispatch_on_value.add([1, 2])
def _(a, my_abc, my_def):
    assert a == [1, 2]
    # Do something

dispatch_on_value.dispatch([1, 2], 'abc', 'def')

You can also pass keyword parameters:

@dispatch_on_value.add([3, 4])
def _(a, my_abc, **kwargs):
    assert 'para1' in kwargs
    # Do something

dispatch_on_value.dispatch([3, 4], 'abc', para1=3)

Methods can also be dispatched

You can dispatch methods on class instances using the add_method decorator:

dispatch_on_value = dv.DispatchOnValue()

class MyClass(object):
    @dispatch_on_value.add_method([1, 2, 3])
    def m1(self, a):
        called[0] = 1
        return 2

    @dispatch_on_value.add_method([4, 5, 6])
    def m2(self, a):
        called[0] = 2
        return 3

my_class = MyClass()

called = [0]

p = [4, 5, 6]
# This will match m2...
dispatch_on_value.dispatch(p) == 3

Matching on dictionaries is either partial or strict

Matching on directories is partial by default. This means dictionaries will match if the key/value pairs in the pattern are matched - any extra pairs in the value passed will be ignored. For example:

@dispatch_on_value.add({'name': 'john', 'age': 32})
def _(a):
    # Do something

# These will match because they contain the minimal dictionary items
dispatch_on_value.dispatch({'name': 'john', 'age': 32})
dispatch_on_value.dispatch({'name': 'john', 'age': 32, 'sex': 'male'})

You can ensure dictionaries have to be exactly the same when matched by using dispatch_strict() rather than dispatch(). For example:

# This will match because it's strict and the pattern is exactly the same
dispatch_on_value.dispatch_strict({'name': 'john', 'age': 32})

# This will not match because the dictionary doesn't match exactly
dispatch_on_value.dispatch_strict({'name': 'john', 'age': 32, 'sex': 'male'})

Author and Contributors

Author: minimind.

Contributors: yurtaev, svisser, mianos.


Python package that provides dispatch on values using pattern matching







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