Backpack ########
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Useful utilities for Python.
Supports Python 2.7+ and 3.2+.
The Collection class provides a fluent, convenient wrapper for working with list of data.
To instantiatte a Collection you can also use the collection() helper.
Available Methods
For the remainder of this documentation, we'll discuss each method available on the Collection class.
Remember, all of these methods may be chained for fluently manipulating the underlying list or dict.
Furthermore, almost every method returns a new Collection instance,
allowing you to preserve the original copy of the collection when necessary.
You may select any method from this table to see an example of its usage:
Methods Listing
.. _all:
all()The all method simply returns the underlying list represented by the collection:
.. code-block:: python
Collection([1, 2, 3]).all()
# [1, 2, 3]
.. _avg:
avg()The avg method returns the average of all items in the collection:
.. code-block:: python
Collection([1, 2, 3, 4, 5]).avg()
# 3
If the collection contains nested objects or dictionaries, you must pass a key to use for determining which values to calculate the average:
.. code-block:: python
collection = Collection([
{'name': 'JavaScript: The Good Parts', 'pages': 176},
{'name': 'JavaScript: The Defnitive Guide', 'pages': 1096}
])
collection.avg('pages')
# 636
.. _chunk:
chunk()The chunk method breaks the collection into multiple, smaller collections of a given size:
.. code-block:: python
collection = Collection([1, 2, 3, 4, 5, 6, 7])
chunks = collection.chunk(4)
chunks.serialize()
# [[1, 2, 3, 4], [5, 6, 7]]
.. _collapse:
collapse()The collapse method collapses a collection of lists into a flat collection:
.. code-block:: python
collection = Collection([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
collapsed = collection.collapse()
collapsed.all()
# [1, 2, 3, 4, 5, 6, 7, 8, 9]
.. _contains:
contains()The contains method determines whether the collection contains a given item:
.. code-block:: python
collection = Collection(['foo', 'bar'])
collection.contains('foo')
# True
You can also use the in keyword:
.. code-block:: python
'foo' in collection
# True
You can also pass a key / value pair to the contains method,
which will determine if the given pair exists in the collection:
.. code-block:: python
collection = Collection([
{'name': 'John', 'id': 1},
{'name': 'Jane', 'id': 2}
])
collection.contains('name', 'Simon')
# False
Finally, you may also pass a callback to the contains method to perform your own truth test:
.. code-block:: python
collection = Collection([1, 2, 3, 4, 5])
collection.contains(lambda item: item > 5)
# False
.. _count:
count()The count method returns the total number of items in the collection:
.. code-block:: python
collection = Collection([1, 2, 3, 4])
collection.count()
# 4
The len function can also be used:
.. code-block:: python
len(collection)
# 4
.. _diff:
diff()The diff method compares the collection against another collection, a list or a dict:
.. code-block:: python
collection = Collection([1, 2, 3, 4, 5])
diff = collection.diff([2, 4, 6, 8])
diff.all()
# [1, 3, 5]
.. _each:
each()The each method iterates over the items in the collection and passes each item to a given callback:
.. code-block:: python
posts.each(lambda post: post.author().save(author))
Return False from your callback to break out of the loop:
.. code-block:: python
posts.each(lambda post: post.author().save(author) if author.name == 'John' else False)
.. _every:
every()The every method creates a new collection consisting of every n-th element:
.. code-block:: python
collection = Collection(['a', 'b', 'c', 'd', 'e', 'f'])
collection.every(4).all()
# ['a', 'e']
You can optionally pass the offset as the second argument:
.. code-block:: python
collection.every(4, 1).all()
# ['b', 'f']
.. _filter:
filter()The filter method filters the collection by a given callback,
keeping only those items that pass a given truth test:
.. code-block:: python
collection = Collection([1, 2, 3, 4])
filtered = collection.filter(lambda item: item > 2)
filtered.all()
# [3, 4]
.. _first:
first()The first method returns the first element in the collection
that passes a given truth test:
.. code-block:: python
collection = Collection([1, 2, 3, 4])
collection.first(lambda item: item > 2)
# 3
You can also call the first method with no arguments
to get the first element in the collection.
If the collection is empty, None is returned:
.. code-block:: python
collection.first()
# 1
.. _flatten:
flatten()The flatten method flattens a multi-dimensional collection into a single dimension:
.. code-block:: python
collection = Collection([1, 2, [3, 4, 5, {'foo': 'bar'}]])
flattened = collection.flatten()
flattened.all()
# [1, 2, 3, 4, 5, 'bar']
.. _forget:
forget()The forget method removes an item from the collection by its key:
.. code-block:: python
collection = Collection([1, 2, 3, 4, 5])
collection.forget(1)
collection.all()
# [1, 3, 4, 5]
.. warning::
Unlike most other collection methods, ``forget`` does not return a new modified collection;
it modifies the collection it is called on.
.. _for_page:
for_page()The for_page method returns a new collection containing
the items that would be present on a given page number:
.. code-block:: python
collection = Collection([1, 2, 3, 4, 5, 6, 7, 8, 9])
chunk = collection.for_page(2, 3)
chunk.all()
# 4, 5, 6
The method requires the page number and the number of items to show per page, respectively.
.. _get:
get()The get method returns the item at a given key. If the key does not exist, None is returned:
.. code-block:: python
collection = Collection([1, 2, 3])
collection.get(3)
# None
You can optionally pass a default value as the second argument:
.. code-block:: python
collection = Collection([1, 2, 3])
collection.get(3, 'default-value')
# default-value
.. _implode:
implode()The implode method joins the items in a collection.
Its arguments depend on the type of items in the collection.
If the collection contains dictionaries or objects, you must pass the key of the attributes you wish to join, and the "glue" string you wish to place between the values:
.. code-block:: python
collection = Collection([
{'account_id': 1, 'product': 'Desk'},
{'account_id': 2, 'product': 'Chair'}
])
collection.implode('product', ', ')
# Desk, Chair
If the collection contains simple strings, simply pass the "glue" as the only argument to the method:
.. code-block:: python
collection = Collection(['foo', 'bar', 'baz'])
collection.implode('-')
# foo-bar-baz
.. _is_empty:
is_empty()The is_empty method returns True if the collection is empty; otherwise, False is returned:
.. code-block:: python
Collection([]).is_empty()
# True
.. _last:
last()The last method returns the last element in the collection that passes a given truth test:
.. code-block:: python
collection = Collection([1, 2, 3, 4])
last = collection.last(lambda item: item < 3)
# 2
You can also call the last method with no arguments to get the last element in the collection.
If the collection is empty, None is returned:
.. code-block:: python
collection.last()
# 4
.. _map:
map()The map method iterates through the collection and passes each value to the given callback.
The callback is free to modify the item and return it, thus forming a new collection of modified items:
.. code-block:: python
collection = Collection([1, 2, 3, 4])
multiplied = collection.map(lambda item: item * 2)
multiplied.all()
# [2, 4, 6, 8]
.. warning::
Like most other collection methods, ``map`` returns a new ``Collection`` instance;
it does not modify the collection it is called on.
If you want to transform the original collection, use the transform_ method.
.. _merge:
merge()The merge method merges the given list into the collection:
.. code-block:: python
collection = Collection(['Desk', 'Chair'])
collection.merge(['Bookcase', 'Door'])
collection.all()
# ['Desk', 'Chair', 'Bookcase', 'Door']
.. warning::
Unlike most other collection methods, ``merge`` does not return a new modified collection;
it modifies the collection it is called on.
.. _pluck:
pluck()The pluck method retrieves all of the collection values for a given key:
.. code-block:: python
collection = Collection([
{'product_id': 1, 'product': 'Desk'},
{'product_id': 2, 'product': 'Chair'}
])
plucked = collection.pluck('product')
plucked.all()
# ['Desk', 'Chair']
You can also specify how you wish the resulting collection to be keyed:
.. code-block:: python
plucked = collection.pluck('name', 'product_id')
plucked
# {1: 'Desk', 2: 'Chair'}
.. _pop:
pop()The pop method removes and returns the last item from the collection:
.. code-block:: python
collection = Collection([1, 2, 3, 4, 5])
collection.pop()
# 5
collection.all()
# [1, 2, 3, 4]
.. _prepend:
prepend()The prepend method adds an item to the beginning of the collection:
.. code-block:: python
collection = Collection([1, 2, 3, 4])
collection.prepend(0)
collection.all()
# [0, 1, 2, 3, 4]
.. _pull:
pull()The pull method removes and returns an item from the collection by its key:
.. code-block:: python
collection = Collection([1, 2, 3, 4])
collection.pull(1)
collection.all()
# [1, 3, 4]
.. _push:
push()/append()The push (or append) method appends an item to the end of the collection:
.. code-block:: python
collection = Collection([1, 2, 3, 4])
collection.push(5)
collection.all()
# [1, 2, 3, 4, 5]
.. _put:
put()The put method sets the given key and value in the collection:
.. code-block:: python
collection = Collection([1, 2, 3, 4])
collection.put(1, 5)
collection.all()
# [1, 5, 3, 4]
.. note::
It is equivalent to:
.. code-block:: python
collection[1] = 5
.. _reduce:
reduce()The reduce method reduces the collection to a single value,
passing the result of each iteration into the subsequent iteration:
.. code-block:: python
collection = Collection([1, 2, 3])
collection.reduce(lambda result, item: (result or 0) + item)
# 6
The value for result on the first iteration is None;
however, you can specify its initial value by passing a second argument to reduce:
.. code-block:: python
collection.reduce(lambda result, item: result + item, 4)
# 10
.. _reject:
reject()The reject method filters the collection using the given callback.
The callback should return True for any items it wishes to remove from the resulting collection:
.. code-block:: python
collection = Collection([1, 2, 3, 4])
filtered = collection.reject(lambda item: item > 2)
filtered.all()
# [1, 2]
For the inverse of reject, see the filter_ method.
.. _reverse:
reverse()The reverse method reverses the order of the collection's items:
.. code-block:: python
collection = Collection([1, 2, 3, 4, 5])
reverse = collection.reverse()
reverse.all()
# [5, 4, 3, 2, 1]
.. _serialize:
serialize()The serialize method converts the collection into a list.
If the collection's values are :ref:ORM models, the models will also be converted to dictionaries:
.. code-block:: python
collection = Collection([User.find(1)])
collection.serialize()
# [{'id': 1, 'name': 'John'}]
.. warning::
``serialize`` also converts all of its nested objects.
If you want to get the underlying items as is, use the all_ method instead.
.. _shift:
shift()The shift method removes and returns the first item from the collection:
.. code-block:: python
collection = Collection([1, 2, 3, 4, 5])
collection.shift()
# 1
collection.all()
# [2, 3, 4, 5]
.. _sort:
sort()The sort method sorts the collection:
.. code-block:: python
collection = Collection([5, 3, 1, 2, 4])
sorted = collection.sort()
sorted.all()
# [1, 2, 3, 4, 5]
.. _sum:
sum()The sum method returns the sum of all items in the collection:
.. code-block:: python
Collection([1, 2, 3, 4, 5]).sum()
# 15
If the collection contains dictionaries or objects, you must pass a key to use for determining which values to sum:
.. code-block:: python
collection = Collection([
{'name': 'JavaScript: The Good Parts', 'pages': 176},
{'name': 'JavaScript: The Defnitive Guide', 'pages': 1096}
])
collection.sum('pages')
# 1272
In addition, you can pass your own callback to determine which values of the collection to sum:
.. code-block:: python
collection = Collection([
{'name': 'Chair', 'colors': ['Black']},
{'name': 'Desk', 'colors': ['Black', 'Mahogany']},
{'name': 'Bookcase', 'colors': ['Red', 'Beige', 'Brown']}
])
collection.sum(lambda product: len(product['colors']))
# 6
.. _take:
take()The take method returns a new collection with the specified number of items:
.. code-block:: python
collection = Collection([0, 1, 2, 3, 4, 5])
chunk = collection.take(3)
chunk.all()
# [0, 1, 2]
You can also pass a negative integer to take the specified amount of items from the end of the collection:
.. code-block:: python
chunk = collection.chunk(-2)
chunk.all()
# [4, 5]
.. _to_json:
to_json()The to_json method converts the collection into JSON:
.. code-block:: python
collection = Collection([{'name': 'Desk', 'price': 200}])
collection.to_json()
# '[{"name": "Desk", "price": 200}]'
.. _transform:
transform()The transform method iterates over the collection and calls the given callback
with each item in the collection.
The items in the collection will be replaced by the values returned by the callback:
.. code-block:: python
collection = Collection([1, 2, 3, 4, 5])
collection.transform(lambda item: item * 2)
collection.all()
# [2, 4, 6, 8, 10]
.. warning::
Unlike most other collection methods, ``transform`` modifies the collection itself.
If you wish to create a new collection instead, use the map_ method.
.. _unique:
unique()The unique method returns all of the unique items in the collection:
.. code-block:: python
collection = Collection([1, 1, 2, 2, 3, 4, 2])
unique = collection.unique()
unique.all()
# [1, 2, 3, 4]
When dealing with dictionaries or objects, you can specify the key used to determine uniqueness:
.. code-block:: python
collection = Collection([
{'name': 'iPhone 6', 'brand': 'Apple', 'type': 'phone'},
{'name': 'iPhone 5', 'brand': 'Apple', 'type': 'phone'},
{'name': 'Apple Watch', 'brand': 'Apple', 'type': 'watch'},
{'name': 'Galaxy S6', 'brand': 'Samsung', 'type': 'phone'},
{'name': 'Galaxy Gear', 'brand': 'Samsung', 'type': 'watch'}
])
unique = collection.unique('brand')
unique.all()
# [
# {'name': 'iPhone 6', 'brand': 'Apple', 'type': 'phone'},
# {'name': 'Galaxy S6', 'brand': 'Samsung', 'type': 'phone'}
# ]
You can also pass your own callback to determine item uniqueness:
.. code-block:: python
unique = collection.unique(lambda item: item['brand'] + item['type'])
unique.all()
# [
# {'name': 'iPhone 6', 'brand': 'Apple', 'type': 'phone'},
# {'name': 'Apple Watch', 'brand': 'Apple', 'type': 'watch'},
# {'name': 'Galaxy S6', 'brand': 'Samsung', 'type': 'phone'},
# {'name': 'Galaxy Gear', 'brand': 'Samsung', 'type': 'watch'}
# ]
.. _where:
where()The where method filters the collection by a given key / value pair:
.. code-block:: python
collection = Collection([
{'name': 'Desk', 'price': 200},
{'name': 'Chair', 'price': 100},
{'name': 'Bookcase', 'price': 150},
{'name': 'Door', 'price': 100},
])
filtered = collection.where('price', 100)
filtered.all()
# [
# {'name': 'Chair', 'price': 100},
# {'name': 'Door', 'price': 100}
# ]
.. _zip:
zip()The zip method merges together the values of the given list
with the values of the collection at the corresponding index:
.. code-block:: python
collection = Collection(['Chair', 'Desk'])
zipped = collection.zip([100, 200])
zipped.all()
# [('Chair', 100), ('Desk', 200)]
FAQs
Useful utilities for Python.
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