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Python 3.6.5 Documentation > "collections" — Container datatypes

"collections" — Container datatypes
***********************************

**Source code:** Lib/collections/__init__.py

======================================================================

This module implements specialized container datatypes providing
alternatives to Python’s general purpose built-in containers, "dict",
"list", "set", and "tuple".

+-----------------------+----------------------------------------------------------------------+
| "namedtuple()" | factory function for creating tuple subclasses with named fields |
+-----------------------+----------------------------------------------------------------------+
| "deque" | list-like container with fast appends and pops on either end |
+-----------------------+----------------------------------------------------------------------+
| "ChainMap" | dict-like class for creating a single view of multiple mappings |
+-----------------------+----------------------------------------------------------------------+
| "Counter" | dict subclass for counting hashable objects |
+-----------------------+----------------------------------------------------------------------+
| "OrderedDict" | dict subclass that remembers the order entries were added |
+-----------------------+----------------------------------------------------------------------+
| "defaultdict" | dict subclass that calls a factory function to supply missing values |
+-----------------------+----------------------------------------------------------------------+
| "UserDict" | wrapper around dictionary objects for easier dict subclassing |
+-----------------------+----------------------------------------------------------------------+
| "UserList" | wrapper around list objects for easier list subclassing |
+-----------------------+----------------------------------------------------------------------+
| "UserString" | wrapper around string objects for easier string subclassing |
+-----------------------+----------------------------------------------------------------------+

Changed in version 3.3: Moved Collections Abstract Base Classes to the
"collections.abc" module. For backwards compatibility, they continue
to be visible in this module as well.

"ChainMap" objects
==================

New in version 3.3.

A "ChainMap" class is provided for quickly linking a number of
mappings so they can be treated as a single unit. It is often much
faster than creating a new dictionary and running multiple "update()"
calls.

The class can be used to simulate nested scopes and is useful in
templating.

class collections.ChainMap(*maps)

A "ChainMap" groups multiple dicts or other mappings together to
create a single, updateable view. If no *maps* are specified, a
single empty dictionary is provided so that a new chain always has
at least one mapping.

The underlying mappings are stored in a list. That list is public
and can be accessed or updated using the *maps* attribute. There
is no other state.

Lookups search the underlying mappings successively until a key is
found. In contrast, writes, updates, and deletions only operate on
the first mapping.

A "ChainMap" incorporates the underlying mappings by reference.
So, if one of the underlying mappings gets updated, those changes
will be reflected in "ChainMap".

All of the usual dictionary methods are supported. In addition,
there is a *maps* attribute, a method for creating new subcontexts,
and a property for accessing all but the first mapping:

maps

A user updateable list of mappings. The list is ordered from
first-searched to last-searched. It is the only stored state
and can be modified to change which mappings are searched. The
list should always contain at least one mapping.

new_child(m=None)

Returns a new "ChainMap" containing a new map followed by all of
the maps in the current instance. If "m" is specified, it
becomes the new map at the front of the list of mappings; if not
specified, an empty dict is used, so that a call to
"d.new_child()" is equivalent to: "ChainMap({}, *d.maps)". This
method is used for creating subcontexts that can be updated
without altering values in any of the parent mappings.

Changed in version 3.4: The optional "m" parameter was added.

parents

Property returning a new "ChainMap" containing all of the maps
in the current instance except the first one. This is useful
for skipping the first map in the search. Use cases are similar
to those for the "nonlocal" keyword used in *nested scopes*.
The use cases also parallel those for the built-in "super()"
function. A reference to "d.parents" is equivalent to:
"ChainMap(*d.maps[1:])".

See also:

* Bag class in Smalltalk.

* Wikipedia entry for Multisets.

* C++ multisets tutorial with examples.

* For mathematical operations on multisets and their use cases,
see *Knuth, Donald. The Art of Computer Programming Volume II,
Section 4.6.3, Exercise 19*.

* To enumerate all distinct multisets of a given size over a given
set of elements, see "itertools.combinations_with_replacement()":

map(Counter, combinations_with_replacement(‘ABC’, 2)) –> AA AB
AC BB BC CC

"deque" objects
===============

class collections.deque([iterable[, maxlen]])

Returns a new deque object initialized left-to-right (using
"append()") with data from *iterable*. If *iterable* is not
specified, the new deque is empty.

Deques are a generalization of stacks and queues (the name is
pronounced “deck” and is short for “double-ended queue”). Deques
support thread-safe, memory efficient appends and pops from either
side of the deque with approximately the same O(1) performance in
either direction.

Though "list" objects support similar operations, they are
optimized for fast fixed-length operations and incur O(n) memory
movement costs for "pop(0)" and "insert(0, v)" operations which
change both the size and position of the underlying data
representation.

If *maxlen* is not specified or is "None", deques may grow to an
arbitrary length. Otherwise, the deque is bounded to the specified
maximum length. Once a bounded length deque is full, when new
items are added, a corresponding number of items are discarded from
the opposite end. Bounded length deques provide functionality
similar to the "tail" filter in Unix. They are also useful for
tracking transactions and other pools of data where only the most
recent activity is of interest.

Deque objects support the following methods:

append(x)

Add *x* to the right side of the deque.

appendleft(x)

Add *x* to the left side of the deque.

clear()

Remove all elements from the deque leaving it with length 0.

copy()

Create a shallow copy of the deque.

New in version 3.5.

count(x)

Count the number of deque elements equal to *x*.

New in version 3.2.

extend(iterable)

Extend the right side of the deque by appending elements from
the iterable argument.

extendleft(iterable)

Extend the left side of the deque by appending elements from
*iterable*. Note, the series of left appends results in
reversing the order of elements in the iterable argument.

index(x[, start[, stop]])

Return the position of *x* in the deque (at or after index
*start* and before index *stop*). Returns the first match or
raises "ValueError" if not found.

New in version 3.5.

insert(i, x)

Insert *x* into the deque at position *i*.

If the insertion would cause a bounded deque to grow beyond
*maxlen*, an "IndexError" is raised.

New in version 3.5.

pop()

Remove and return an element from the right side of the deque.
If no elements are present, raises an "IndexError".

popleft()

Remove and return an element from the left side of the deque. If
no elements are present, raises an "IndexError".

remove(value)

Remove the first occurrence of *value*. If not found, raises a
"ValueError".

reverse()

Reverse the elements of the deque in-place and then return
"None".

New in version 3.2.

rotate(n=1)

Rotate the deque *n* steps to the right. If *n* is negative,
rotate to the left.

When the deque is not empty, rotating one step to the right is
equivalent to "d.appendleft(d.pop())", and rotating one step to
the left is equivalent to "d.append(d.popleft())".

Deque objects also provide one read-only attribute:

maxlen

Maximum size of a deque or "None" if unbounded.

New in version 3.1.

In addition to the above, deques support iteration, pickling,
"len(d)", "reversed(d)", "copy.copy(d)", "copy.deepcopy(d)",
membership testing with the "in" operator, and subscript references
such as "d[-1]". Indexed access is O(1) at both ends but slows to
O(n) in the middle. For fast random access, use lists instead.

Starting in version 3.5, deques support "__add__()", "__mul__()", and
"__imul__()".

Example:

>>> from collections import deque
>>> d = deque('ghi') # make a new deque with three items
>>> for elem in d: # iterate over the deque's elements
... print(elem.upper())
G
H
I

>>> d.append('j') # add a new entry to the right side
>>> d.appendleft('f') # add a new entry to the left side
>>> d # show the representation of the deque
deque(['f', 'g', 'h', 'i', 'j'])

>>> d.pop() # return and remove the rightmost item
'j'
>>> d.popleft() # return and remove the leftmost item
'f'
>>> list(d) # list the contents of the deque
['g', 'h', 'i']
>>> d[0] # peek at leftmost item
'g'
>>> d[-1] # peek at rightmost item
'i'

>>> list(reversed(d)) # list the contents of a deque in reverse
['i', 'h', 'g']
>>> 'h' in d # search the deque
True
>>> d.extend('jkl') # add multiple elements at once
>>> d
deque(['g', 'h', 'i', 'j', 'k', 'l'])
>>> d.rotate(1) # right rotation
>>> d
deque(['l', 'g', 'h', 'i', 'j', 'k'])
>>> d.rotate(-1) # left rotation
>>> d
deque(['g', 'h', 'i', 'j', 'k', 'l'])

>>> deque(reversed(d)) # make a new deque in reverse order
deque(['l', 'k', 'j', 'i', 'h', 'g'])
>>> d.clear() # empty the deque
>>> d.pop() # cannot pop from an empty deque
Traceback (most recent call last):
File "<pyshell#6>", line 1, in -toplevel-
d.pop()
IndexError: pop from an empty deque

>>> d.extendleft('abc') # extendleft() reverses the input order
>>> d
deque(['c', 'b', 'a'])

"deque" Recipes
---------------

This section shows various approaches to working with deques.

Bounded length deques provide functionality similar to the "tail"
filter in Unix:

def tail(filename, n=10):
'Return the last n lines of a file'
with open(filename) as f:
return deque(f, n)

Another approach to using deques is to maintain a sequence of recently
added elements by appending to the right and popping to the left:

def moving_average(iterable, n=3):
# moving_average([40, 30, 50, 46, 39, 44]) --> 40.0 42.0 45.0 43.0
# http://en.wikipedia.org/wiki/Moving_average
it = iter(iterable)
d = deque(itertools.islice(it, n-1))
d.appendleft(0)
s = sum(d)
for elem in it:
s += elem - d.popleft()
d.append(elem)
yield s / n

The "rotate()" method provides a way to implement "deque" slicing and
deletion. For example, a pure Python implementation of "del d[n]"
relies on the "rotate()" method to position elements to be popped:

def delete_nth(d, n):
d.rotate(-n)
d.popleft()
d.rotate(n)

To implement "deque" slicing, use a similar approach applying
"rotate()" to bring a target element to the left side of the deque.
Remove old entries with "popleft()", add new entries with "extend()",
and then reverse the rotation. With minor variations on that approach,
it is easy to implement Forth style stack manipulations such as "dup",
"drop", "swap", "over", "pick", "rot", and "roll".

"defaultdict" objects
=====================

class collections.defaultdict([default_factory[, ...]])

Returns a new dictionary-like object. "defaultdict" is a subclass
of the built-in "dict" class. It overrides one method and adds one
writable instance variable. The remaining functionality is the
same as for the "dict" class and is not documented here.

The first argument provides the initial value for the
"default_factory" attribute; it defaults to "None". All remaining
arguments are treated the same as if they were passed to the "dict"
constructor, including keyword arguments.

"defaultdict" objects support the following method in addition to
the standard "dict" operations:

__missing__(key)

If the "default_factory" attribute is "None", this raises a
"KeyError" exception with the *key* as argument.

If "default_factory" is not "None", it is called without
arguments to provide a default value for the given *key*, this
value is inserted in the dictionary for the *key*, and returned.

If calling "default_factory" raises an exception this exception
is propagated unchanged.

This method is called by the "__getitem__()" method of the
"dict" class when the requested key is not found; whatever it
returns or raises is then returned or raised by "__getitem__()".

Note that "__missing__()" is *not* called for any operations
besides "__getitem__()". This means that "get()" will, like
normal dictionaries, return "None" as a default rather than
using "default_factory".

"defaultdict" objects support the following instance variable:

default_factory

This attribute is used by the "__missing__()" method; it is
initialized from the first argument to the constructor, if
present, or to "None", if absent.

"defaultdict" Examples
----------------------

Using "list" as the "default_factory", it is easy to group a sequence
of key-value pairs into a dictionary of lists:

>>> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
>>> d = defaultdict(list)
>>> for k, v in s:
... d[k].append(v)
...
>>> sorted(d.items())
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

When each key is encountered for the first time, it is not already in
the mapping; so an entry is automatically created using the
"default_factory" function which returns an empty "list". The
"list.append()" operation then attaches the value to the new list.
When keys are encountered again, the look-up proceeds normally
(returning the list for that key) and the "list.append()" operation
adds another value to the list. This technique is simpler and faster
than an equivalent technique using "dict.setdefault()":

>>> d = {}
>>> for k, v in s:
... d.setdefault(k, []).append(v)
...
>>> sorted(d.items())
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

Setting the "default_factory" to "int" makes the "defaultdict" useful
for counting (like a bag or multiset in other languages):

>>> s = 'mississippi'
>>> d = defaultdict(int)
>>> for k in s:
... d[k] += 1
...
>>> sorted(d.items())
[('i', 4), ('m', 1), ('p', 2), ('s', 4)]

When a letter is first encountered, it is missing from the mapping, so
the "default_factory" function calls "int()" to supply a default count
of zero. The increment operation then builds up the count for each
letter.

The function "int()" which always returns zero is just a special case
of constant functions. A faster and more flexible way to create
constant functions is to use a lambda function which can supply any
constant value (not just zero):

>>> def constant_factory(value):
... return lambda: value
>>> d = defaultdict(constant_factory('<missing>'))
>>> d.update(name='John', action='ran')
>>> '%(name)s %(action)s to %(object)s' % d
'John ran to <missing>'

Setting the "default_factory" to "set" makes the "defaultdict" useful
for building a dictionary of sets:

>>> s = [('red', 1), ('blue', 2), ('red', 3), ('blue', 4), ('red', 1), ('blue', 4)]
>>> d = defaultdict(set)
>>> for k, v in s:
... d[k].add(v)
...
>>> sorted(d.items())
[('blue', {2, 4}), ('red', {1, 3})]

"namedtuple()" Factory Function for Tuples with Named Fields
============================================================

Named tuples assign meaning to each position in a tuple and allow for
more readable, self-documenting code. They can be used wherever
regular tuples are used, and they add the ability to access fields by
name instead of position index.

collections.namedtuple(typename, field_names, *, verbose=False, rename=False, module=None)

Returns a new tuple subclass named *typename*. The new subclass is
used to create tuple-like objects that have fields accessible by
attribute lookup as well as being indexable and iterable.
Instances of the subclass also have a helpful docstring (with
typename and field_names) and a helpful "__repr__()" method which
lists the tuple contents in a "name=value" format.

The *field_names* are a sequence of strings such as "['x', 'y']".
Alternatively, *field_names* can be a single string with each
fieldname separated by whitespace and/or commas, for example "'x
y'" or "'x, y'".

Any valid Python identifier may be used for a fieldname except for
names starting with an underscore. Valid identifiers consist of
letters, digits, and underscores but do not start with a digit or
underscore and cannot be a "keyword" such as *class*, *for*,
*return*, *global*, *pass*, or *raise*.

If *rename* is true, invalid fieldnames are automatically replaced
with positional names. For example, "['abc', 'def', 'ghi', 'abc']"
is converted to "['abc', '_1', 'ghi', '_3']", eliminating the
keyword "def" and the duplicate fieldname "abc".

If *verbose* is true, the class definition is printed after it is
built. This option is outdated; instead, it is simpler to print
the "_source" attribute.

If *module* is defined, the "__module__" attribute of the named
tuple is set to that value.

Named tuple instances do not have per-instance dictionaries, so
they are lightweight and require no more memory than regular
tuples.

Changed in version 3.1: Added support for *rename*.

Changed in version 3.6: The *verbose* and *rename* parameters
became keyword-only arguments.

Changed in version 3.6: Added the *module* parameter.

>>> # Basic example
>>> Point = namedtuple('Point', ['x', 'y'])
>>> p = Point(11, y=22) # instantiate with positional or keyword arguments
>>> p[0] + p[1] # indexable like the plain tuple (11, 22)
33
>>> x, y = p # unpack like a regular tuple
>>> x, y
(11, 22)
>>> p.x + p.y # fields also accessible by name
33
>>> p # readable __repr__ with a name=value style
Point(x=11, y=22)

Named tuples are especially useful for assigning field names to result
tuples returned by the "csv" or "sqlite3" modules:

EmployeeRecord = namedtuple('EmployeeRecord', 'name, age, title, department, paygrade')

import csv
for emp in map(EmployeeRecord._make, csv.reader(open("employees.csv", "rb"))):
print(emp.name, emp.title)

import sqlite3
conn = sqlite3.connect('/companydata')
cursor = conn.cursor()
cursor.execute('SELECT name, age, title, department, paygrade FROM employees')
for emp in map(EmployeeRecord._make, cursor.fetchall()):
print(emp.name, emp.title)

In addition to the methods inherited from tuples, named tuples support
three additional methods and two attributes. To prevent conflicts
with field names, the method and attribute names start with an
underscore.

classmethod somenamedtuple._make(iterable)

Class method that makes a new instance from an existing sequence or
iterable.

>>> t = [11, 22]
>>> Point._make(t)
Point(x=11, y=22)

somenamedtuple._asdict()

Return a new "OrderedDict" which maps field names to their
corresponding values:

>>> p = Point(x=11, y=22)
>>> p._asdict()
OrderedDict([('x', 11), ('y', 22)])

Changed in version 3.1: Returns an "OrderedDict" instead of a
regular "dict".

somenamedtuple._replace(**kwargs)

Return a new instance of the named tuple replacing specified fields
with new values:

>>> p = Point(x=11, y=22)
>>> p._replace(x=33)
Point(x=33, y=22)

>>> for partnum, record in inventory.items():
... inventory[partnum] = record._replace(price=newprices[partnum], timestamp=time.now())

somenamedtuple._source

A string with the pure Python source code used to create the named
tuple class. The source makes the named tuple self-documenting. It
can be printed, executed using "exec()", or saved to a file and
imported.

New in version 3.3.

somenamedtuple._fields

Tuple of strings listing the field names. Useful for introspection
and for creating new named tuple types from existing named tuples.

>>> p._fields # view the field names
('x', 'y')

>>> Color = namedtuple('Color', 'red green blue')
>>> Pixel = namedtuple('Pixel', Point._fields + Color._fields)
>>> Pixel(11, 22, 128, 255, 0)
Pixel(x=11, y=22, red=128, green=255, blue=0)

To retrieve a field whose name is stored in a string, use the
"getattr()" function:

>>> getattr(p, 'x')
11

To convert a dictionary to a named tuple, use the double-star-operator
(as described in Unpacking Argument Lists):

>>> d = {'x': 11, 'y': 22}
>>> Point(**d)
Point(x=11, y=22)

Since a named tuple is a regular Python class, it is easy to add or
change functionality with a subclass. Here is how to add a calculated
field and a fixed-width print format:

>>> class Point(namedtuple('Point', ['x', 'y'])):
... __slots__ = ()
... @property
... def hypot(self):
... return (self.x ** 2 + self.y ** 2) ** 0.5
... def __str__(self):
... return 'Point: x=%6.3f y=%6.3f hypot=%6.3f' % (self.x, self.y, self.hypot)

>>> for p in Point(3, 4), Point(14, 5/7):
... print(p)
Point: x= 3.000 y= 4.000 hypot= 5.000
Point: x=14.000 y= 0.714 hypot=14.018

The subclass shown above sets "__slots__" to an empty tuple. This
helps keep memory requirements low by preventing the creation of
instance dictionaries.

Subclassing is not useful for adding new, stored fields. Instead,
simply create a new named tuple type from the "_fields" attribute:

>>> Point3D = namedtuple('Point3D', Point._fields + ('z',))

Docstrings can be customized by making direct assignments to the
"__doc__" fields:

>>> Book = namedtuple('Book', ['id', 'title', 'authors'])
>>> Book.__doc__ += ': Hardcover book in active collection'
>>> Book.id.__doc__ = '13-digit ISBN'
>>> Book.title.__doc__ = 'Title of first printing'
>>> Book.authors.__doc__ = 'List of authors sorted by last name'

Changed in version 3.5: Property docstrings became writeable.

Default values can be implemented by using "_replace()" to customize a
prototype instance:

>>> Account = namedtuple('Account', 'owner balance transaction_count')
>>> default_account = Account('<owner name>', 0.0, 0)
>>> johns_account = default_account._replace(owner='John')
>>> janes_account = default_account._replace(owner='Jane')

See also:

* Recipe for named tuple abstract base class with a metaclass mix-
in by Jan Kaliszewski. Besides providing an *abstract base class*
for named tuples, it also supports an alternate *metaclass*-based
constructor that is convenient for use cases where named tuples
are being subclassed.

* See "types.SimpleNamespace()" for a mutable namespace based on
an underlying dictionary instead of a tuple.

* See "typing.NamedTuple()" for a way to add type hints for named
tuples.

"OrderedDict" objects
=====================

Ordered dictionaries are just like regular dictionaries but they
remember the order that items were inserted. When iterating over an
ordered dictionary, the items are returned in the order their keys
were first added.

class collections.OrderedDict([items])

Return an instance of a dict subclass, supporting the usual "dict"
methods. An *OrderedDict* is a dict that remembers the order that
keys were first inserted. If a new entry overwrites an existing
entry, the original insertion position is left unchanged. Deleting
an entry and reinserting it will move it to the end.

New in version 3.1.

popitem(last=True)

The "popitem()" method for ordered dictionaries returns and
removes a (key, value) pair. The pairs are returned in LIFO
(last-in, first-out) order if *last* is true or FIFO (first-in,
first-out) order if false.

move_to_end(key, last=True)

Move an existing *key* to either end of an ordered dictionary.
The item is moved to the right end if *last* is true (the
default) or to the beginning if *last* is false. Raises
"KeyError" if the *key* does not exist:

>>> d = OrderedDict.fromkeys('abcde')
>>> d.move_to_end('b')
>>> ''.join(d.keys())
'acdeb'
>>> d.move_to_end('b', last=False)
>>> ''.join(d.keys())
'bacde'

New in version 3.2.

In addition to the usual mapping methods, ordered dictionaries also
support reverse iteration using "reversed()".

Equality tests between "OrderedDict" objects are order-sensitive and
are implemented as "list(od1.items())==list(od2.items())". Equality
tests between "OrderedDict" objects and other "Mapping" objects are
order-insensitive like regular dictionaries. This allows
"OrderedDict" objects to be substituted anywhere a regular dictionary
is used.

Changed in version 3.5: The items, keys, and values *views* of
"OrderedDict" now support reverse iteration using "reversed()".

Changed in version 3.6: With the acceptance of **PEP 468**, order is
retained for keyword arguments passed to the "OrderedDict" constructor
and its "update()" method.

"OrderedDict" Examples and Recipes
----------------------------------

Since an ordered dictionary remembers its insertion order, it can be
used in conjunction with sorting to make a sorted dictionary:

>>> # regular unsorted dictionary
>>> d = {'banana': 3, 'apple': 4, 'pear': 1, 'orange': 2}

>>> # dictionary sorted by key
>>> OrderedDict(sorted(d.items(), key=lambda t: t[0]))
OrderedDict([('apple', 4), ('banana', 3), ('orange', 2), ('pear', 1)])

>>> # dictionary sorted by value
>>> OrderedDict(sorted(d.items(), key=lambda t: t[1]))
OrderedDict([('pear', 1), ('orange', 2), ('banana', 3), ('apple', 4)])

>>> # dictionary sorted by length of the key string
>>> OrderedDict(sorted(d.items(), key=lambda t: len(t[0])))
OrderedDict([('pear', 1), ('apple', 4), ('orange', 2), ('banana', 3)])

The new sorted dictionaries maintain their sort order when entries are
deleted. But when new keys are added, the keys are appended to the
end and the sort is not maintained.

It is also straight-forward to create an ordered dictionary variant
that remembers the order the keys were *last* inserted. If a new entry
overwrites an existing entry, the original insertion position is
changed and moved to the end:

class LastUpdatedOrderedDict(OrderedDict):
'Store items in the order the keys were last added'

def __setitem__(self, key, value):
if key in self:
del self[key]
OrderedDict.__setitem__(self, key, value)

An ordered dictionary can be combined with the "Counter" class so that
the counter remembers the order elements are first encountered:

class OrderedCounter(Counter, OrderedDict):
'Counter that remembers the order elements are first encountered'

def __repr__(self):
return '%s(%r)' % (self.__class__.__name__, OrderedDict(self))

def __reduce__(self):
return self.__class__, (OrderedDict(self),)

"UserDict" objects
==================

The class, "UserDict" acts as a wrapper around dictionary objects. The
need for this class has been partially supplanted by the ability to
subclass directly from "dict"; however, this class can be easier to
work with because the underlying dictionary is accessible as an
attribute.

class collections.UserDict([initialdata])

Class that simulates a dictionary. The instance’s contents are
kept in a regular dictionary, which is accessible via the "data"
attribute of "UserDict" instances. If *initialdata* is provided,
"data" is initialized with its contents; note that a reference to
*initialdata* will not be kept, allowing it be used for other
purposes.

In addition to supporting the methods and operations of mappings,
"UserDict" instances provide the following attribute:

data

A real dictionary used to store the contents of the "UserDict"
class.

"UserList" objects
==================

This class acts as a wrapper around list objects. It is a useful base
class for your own list-like classes which can inherit from them and
override existing methods or add new ones. In this way, one can add
new behaviors to lists.

The need for this class has been partially supplanted by the ability
to subclass directly from "list"; however, this class can be easier to
work with because the underlying list is accessible as an attribute.

class collections.UserList([list])

Class that simulates a list. The instance’s contents are kept in a
regular list, which is accessible via the "data" attribute of
"UserList" instances. The instance’s contents are initially set to
a copy of *list*, defaulting to the empty list "[]". *list* can be
any iterable, for example a real Python list or a "UserList"
object.

In addition to supporting the methods and operations of mutable
sequences, "UserList" instances provide the following attribute:

data

A real "list" object used to store the contents of the
"UserList" class.

**Subclassing requirements:** Subclasses of "UserList" are expected to
offer a constructor which can be called with either no arguments or
one argument. List operations which return a new sequence attempt to
create an instance of the actual implementation class. To do so, it
assumes that the constructor can be called with a single parameter,
which is a sequence object used as a data source.

If a derived class does not wish to comply with this requirement, all
of the special methods supported by this class will need to be
overridden; please consult the sources for information about the
methods which need to be provided in that case.

"UserString" objects
====================

The class, "UserString" acts as a wrapper around string objects. The
need for this class has been partially supplanted by the ability to
subclass directly from "str"; however, this class can be easier to
work with because the underlying string is accessible as an attribute.

class collections.UserString([sequence])

Class that simulates a string or a Unicode string object. The
instance’s content is kept in a regular string object, which is
accessible via the "data" attribute of "UserString" instances. The
instance’s contents are initially set to a copy of *sequence*. The
*sequence* can be an instance of "bytes", "str", "UserString" (or a
subclass) or an arbitrary sequence which can be converted into a
string using the built-in "str()" function.

Changed in version 3.5: New methods "__getnewargs__", "__rmod__",
"casefold", "format_map", "isprintable", and "maketrans".

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