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A collections pipeline tool for iterable objects with support for iteration with `dict.items`

Project description

This package is inspired heavily by Martin Fowler’s Collection Pipeline pattern, and also by my first interaction with this pattern–Laravel’s Collections in PHP. This is meant to be a versatile and extensible implementation of the Collection Pipeline pattern that leverages Python’s builtin functionality to provide a clearer, more legible implementation than usage of builtins alone provides. In other words, this package is born of the belief that this is more difficult to decipher:

foo = list(filter(lambda y: y > 4, map(lambda x: x * 2, [1, 2, 3, 4])))

Than this:

foo = collect([1, 2, 3, 4]).map(lambda x: x * 2) \
    .filter(lambda y: y > 4) \

HOWEVER if you’re only going to be working with Sequences, if you would like to connect directly to a data source, if you are concerned with maintaining your data in an immutable chain, or if you prefer the syntax of Linq or Scala (and don’t feel like taking the time to implement it yourself with the tools provided in this package), I would highly suggest you use PyFunctional. It’s a much more mature, more well conceived, and time tested package that provides the same object oriented pipeline interface. Where these packages differ is in their intentions. As stated clearly in this Github issue, PyFunctional is built around these concepts:

  • Constructing a lineage chain (functions that take an iterable and return one, or actions)
  • Applying it to a base collection via the iterator interface, the base collection is also forced to be a list.

Iterable Collections was made with Dicts and other Mappings in mind–which is not to purport any superiority, just to state they were conceived for different purposes. Because of the desire to work with Mappings, the desire for an easily extendable interface, and the desire leverage builtins in a pipeline interface, I decided there was room for a similar package.

Python 2.7 Support

There is currently no support for Python 2.7. That may be addressed in the future, but at the moment it is not a priority.


Basic usage of transformational methods:

from iterable_collections import collect

# map
collect([1, 2, 3, 4]).map(lambda x: x + 2) \
    .list() # [3, 4, 5, 6]

# filter
collect([1, 2, 3, 4]).filter(lambda x: x > 2) \
    .list() # [3, 4]

# groupby
collect([{'a': 1, 'b': 2}, {'a': 1, 'b': 3}, {'a': 1, 'b': 2}]).sorted(key=lambda x: x['b']) \
    .groupby(key=lambda x: x['b'])
    .map(lambda x: (x[0], list(x[1]))) # We're using map here because the results of groupby are in iterators
    .list() # [(2, [{'a': 1, 'b': 2}, {'a': 1, 'b': 2}]), (3, [{'a': 1, 'b': 3}])]

# reduce
collect([1, 2, 3, 4]).reduce(lambda t, x: t + x) # 10

# flatten
collect([[1, 2], [3, 4]]).flatten() \
    .list() # [1, 2, 3, 4]

There are two approaches that can be used with Mappings: the *_items and *_nt_items methods, such as map_items and map_nt_items. The *_items methods implicitly call dict.items on the Collection.iterable property which results in a Mapping View containing key value pairs. These methods accept a two argument function instead of the usual one argument function used by map:

collect({'a': 1, 'b': 2, 'c': 3, 'd': 4}).map_items(lambda k, v: (k, v + 2)) \
    .dict() # {'a': 3, 'b': 4, 'c': 5, 'd': 6}

Also available are the *_nt_items methods. These methods also implicitly call dict.items, but instead of spreading the keys and values into two arguments, they create a namedtuple that has key and value attributes:

collect({'a': 1, 'b': 2, 'c': 3, 'd': 4}).map_nt_items(lambda x: (x.key, x.value + 2)) \
    .dict() # {'a': 3, 'b': 4, 'c': 5, 'd': 6}

There *_items and *_nt_items available for the transformational methods: filter, map, and reduce. Additionally the first and last methods have *_items and *_nt_items versions:

# first
collect([1, 2, 3, 4]).first() # 1

# last
collect([1, 2, 3, 4]).last() # 4

# first_item
collect({'a': 1, 'b': 2, 'c': 3, 'd': 4}).first_item() # ('a', 1)

# last_item
collect({'a': 1, 'b': 2, 'c': 3, 'd': 4}).last_item() # ('d', 4)

# first_nt_item
collect({'a': 1, 'b': 2, 'c': 3, 'd': 4}).first_nt_item() # DictItem(key='a', value=1)

# last_nt_item
collect({'a': 1, 'b': 2, 'c': 3, 'd': 4}).last_nt_item() # DictItem(key='d', value=4)


Each method on the Collection object is defined by a MethodStrategy. These objects utilize a Callable object and a series of other strategies to compose all the functionality encapsulated by a method. As defined in the MethodStrategy class, the process of encapsulating the functionality of a method can be characterized with six phases:

1. Pre-Processing: A series of operations are performed on the Collection.iterable property. Typically iterable is converted into some desired format or type.

2. Argument Formatting: Format the arguments passed to the method. Also in this phase, typically arguments are formatted into some desired format or type.

  1. Argument Binding: Bind the arguments to desired positions in the Callable’s signature.
  2. Execution: The Callable object is called, performing the core functionality of the method–may result in errors.

5. Result Handling: Determine what should be done with the result of the Callable. Basic behavior is either storing the result to the Collection.iterable property or preserving it’s original value.

6. Return Value: Determine what value should be returned from the method. Basic behavior is either the result of the Callable or the current Collection instance.

In order to determine the behaviors invoked in each of these phases, MethodStrategy is initialized with the method’s name as a str, a Callable object, and a number of objects that implement interfaces which abstract the various phases.

  • PreProcessingStrategyInterface
  • ArgumentFormattingStrategyInterface
  • ArgumentBindingStrategyInterface
  • ErrorHandlingStrategyInterface (Handles any errors that occur during the Execution phase)
  • ResultStrategyInterface
  • ReturnValueStrategyInterface

An illustrative example of how these would be implemented can be seen in the definition of Collection.first_item:

    'first_item', # name of the method
    operator.itemgetter(0), # the Callable object
    StoreIterableStrategy(), # Result Handling phase
    ReturnResultStrategy(), # Return Value phase
    PartialIterableBindingStrategy(), # Argument Binding phase
    UnformattedArgumentFormattingStrategy(), # Argument Formatting phase
        {'name': 'items', 'args': (), 'kwargs': {}},
        {'name': 'tuple', 'args': (), 'kwargs': {'store': True}}
    )), # Pre-Processing phase
    GetItemErrorHandlingStrategy() # Error handling during Execution phase

All of these interfaces can be implemented in several different ways. The implementations found in the iterable_collections.strategy module are only those needed for the base cannon of methods. Collection.first_item is a good demonstration of how to leverage Python’s builtin functionality. However if the Callable argument needs custom behavior, MethodStrategy can be extended, as in ConcatMethodStrategy:

class ConcatMethodStrategy(MethodStrategy):
    def __init__(
            getattr(self, name),

    def concat(self, instance, *args):
        for a in args:
            if not isinstance(a, Iterable):
                raise TypeError('Must an iterable type.')
            if isinstance(a, (Set, Sequence, MappingView)):
            if isinstance(a, Mapping):
            if isinstance(a, Iterator):
        return instance.iterable

    def concat_dict(self, iterable, other):
        return iterable

    def concat_iter(self, iterable, other):
        return itertools.chain(iterable, other)

    def concat_seq(self, iterable, other):
        return operator.concat(iterable, other)

Different types are concatenated in different ways, so methods are defined for Mappings, Iterators, and Sequences. A universal method concat determines which of these methods to call based on the type of each argument. Notice though that the methods are being called on instance which is the current instance of the Collection object (i.e. self in Collection.). By calling these on the instance, any pre-processing, argument formatting, etc… will take place. This would not happen if these methods were called in this way, self.concat_dict(instance.iterable, a).

However to call methods this way, they must have been defined before the Collection object was instantiated. Collection is initialized with an Iterable and a StrategyDict, which is a Mapping of str method names to objects implementing the MethodStrategyInterface. The included StrategyDict is created using DefaultMethodStrategyFactory defined in iterable_collections.factory. The collect function creates a factory object and then uses it to create the StrategyDict injected into Collection. Below is an excerpt of DefaultMethodStrategyFactory:

class DefaultMethodStrategyFactory(MethodStrategyFactoryInterface):
    def create(self):
        return { s for s in self.get_strategies()} # Converts the Tuple of MethodStrategyInterface object
                                                          # into a Dict.

    def get_strategies(self): # Returns a Tuple of MethodStrategyInterface objects
        return (
            ChunksMethodStrategy( # A child class of MethodStrategy that breaks iterables into chunks.


Depending on how this received, future plans may include:

  • Extensibility through configurations.
  • Event driven extensibility.
  • Python 2.7 support. (Though not likely unless there is demand.)
  • I don’t know… You got any ideas?

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