Python Dictionaries

7/25/2019 Python Dictionaries

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https://jeffknupp.com/blog/2015/08/30/python-dictionaries/ 1

Everything I KnowAbout Python...

The personal blog of author, speaker, tutor, and professional

software engineer Jeff Knupp

Python DictionariesAside: one thing I dislike about the official Python documentation is

that only a small percentage of entries have example code. We should

change that...)

One of the keys to becoming a better Python programmer is to have a solidgrasp of Python's built-in data structures. Using the structured format below,

today you'll learn what a dict is, when to use it, and see example code of

allof its member functions. I have some other data structures in the works,

so this may turn into a little series.

Dictionary

AKA"Associate Array", "Map", "Hash Map", "Unordered Map"

Librarybuilt-in

DescriptionContains a series of key -> value mappings where the "key" is of any type

that is hashable(meaning it has both a __eq__() and a __hash__()

method). The "value" may be of any type and value types need not be

homogeneous.

That means, for example, we can have a dictionary where some keys map to

strings and others to ints. Probably not a great idea in practice, but there's

nothing stopping you from doing it.
https://jeffknupp.com/blog/2015/08/30/python-dictionarieshttps://jeffknupp.com/blog/2015/08/30/python-dictionarieshttps://jeffknupp.com/blog/2015/08/30/python-dictionaries


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What Makes it SpecialThe conceptual implementation is that of a hash table, so checks for

existence are quite fast. That means we can determine if a specific key is

present in the dictionary without needing to examine every element (which

gets slower as the dictionary gets bigger). The Python interpreter can just go

to the location key "should be" at (if it's in the dictionary) and see if key is

actually there.

Construction

Literal{} : pair of braces for empty dictionary

{1:2, 3:4} : comma-separated list of the form key: value

enclosed by braces

Constructor

dict(one=2, three=4) : using dict() with keyword arguments

mapping keys to values (where one and two are valid identifiers)

dict([(1, 2), (3, 4)]) : using dict() with an iterable

containing iterables with exactly two objects, the key and value

dict(zip([1, 3], [2, 4])) : using dict() with two iterables of

equal length; the first contains a list of keys and the second contains

their associated values.dict({1:2, 3:4}) : using dict() with the literal form as an

argument. This is silly. Why would you want this?

Mutabilitymutable

Orderingundefined

When to Use ItWhen describing what you want to do, if you use the word "map" (or

"match"), chances are good you need a dictionary. Use whenever a mapping

from a key to a value is required.

Example Usage


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state_capitals={

'New York':'Albany',

'New Jersey':'Trenton',

}

"New York" is a keyand "Albany" is a value. This allows us to retrieve a

state's capital if we have the state's name by doing capital =

state_capitals[state]

How Notto Use ItRemember, the great thing about dictionaries is we can find a value instantly,

without needing to search through the whole dictionary manually, using the

form value = my_dict['key'] or value = my_dict.get('key',

None) .

If you're searching for a value in a dictionary and you use a for loop,

you're doing it wrong. Stop, go back, and read the previous statement.

All too often in beginner code I see the equivalent of the following (continuing

the previous example):

state_im_looking_for ='New Jersey'

my_capital =''

forstate in state_capitals:

ifstate ==state_im_looking_for:my_capital =state_capitals[state]

Or like this:

state_im_looking_for ='New Jersey'

my_capital =''

forstate, capital in state_capitals.items():

ifstate ==state_im_looking_for:

my_capital =capital

Methods and Uses

d.clear()Remove all entries in d

Returns


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N/A

Raises

N/A

Examples

Delete all items in a dictionary

d.clear()

d.copy()Make a shallowcopy of d . The dictionary returned by d.copy() will have

the same references as d , not copies of the items.

Returns

A new dict , representing a shallowcopy of d

Raises

N/A

Examples

Create copy of a dictionary

d ={1:'a', 2:'b', 3:'c'}

copied_dict =d.copy()

copied_dict # {1:'a', 2:'b', 3:'c'}

d[1] ='z'

copied_dict # {1:'a', 2:'b', 3:'c'}

del k[d]Used to remove a value from a dictionary

ReturnsN/A

Raises

KeyError if key is not in dictionary

Examples

Delete entry with key 'hello'


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my_dictionary ={'hello':1, 'goodbye':2}

del my_dictionary['hello']

print(my_dictionary)

# {'goodbye': 2}

dict.fromkeys(seq[, value])Create a new dictionary with the same keys as seq . If value is provided,

each item's value is set to value . If value is not set, all item values are

set to None

Returns

N/A

Raises

N/A

Examples

Create a dictionary from a list with all values initialized to 0

my_list =[1, 2, 3]

my_dictionary =dict.fromkeys(my_list, 0)

my_dictionary # {1:0, 2:0, 3:0}

Create a dictionary from a dictionary with all values automatically initialized

to None

my_dictionary ={1:1, 2:2, 3:3}

new_dictionary =dict.fromkeys(my_dictionary)

my_dictionary # {1:None, 2:None, 3:None}

d.get(key[, default)Used to retrieve the value associated with key key . The value of default

is returned if key is not in d (rather than raising a KeyError ). The

default value of default is None .

Returns

Roughly equivalent to:


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def get(key, default=None):

ifkey in d:

returnd[k]

else:

returndefault

Raises

N/A

Examples

Get a key's value or None if the key isn't present

{1:'a', 2:'b'}.get(3)

k in dUsed to iterate over the keys, values, or both of the dictionary.

Returns

N/A

Raises

N/A

Examples

Iterate over keys

forkey in my_dictionary:

Iterate over (key, value) tuples

forkey, value in my_dictionary.items():

Iterate over values

forvalue in my_dictionary.values():

Check for existence

haystack ={}

# ...

if'needle' in haystack:

iter(d)


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Used to iterate over the keys of d

Returns

An iterator which iterates over the keys of d

Raises

StopIteration when d has no more keys

Examples

Iterate over keys

forkey in my_dictionary:

d[key]Used to access the value corresponding to the key key in d .

Returns

Value associated with the key (heterogeneous)

Raises

KeyError when key is not a member of d .

Examples

capitals ={'New York':'Albany'}`capital_of_ny =capitals['New York']`

print capital_of_ny`

'Albany'

len(d)Used to determine the number of entries in a dictionary

Returns

Length of dictionary d

Raises

N/A

Examples

print 'dictionary has {} entries'.format(len(d))


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k not in dUsed for negative existence check. Equivalent to not key in value

Returns

True if key is not in value , False otherwise

RaisesN/A

Examples

Check for negative existence

haystack ={}

# ...

if'needle' not in haystack:

d.keys()Iterate over the keys in a dictionary

Returns

An iterable over all of the keysin d (in an unspecified order)

Raises

StopIteration when d has no more keys

Examples

Iterate over keys:

forkey in d.keys():

d.values()

Iterate over the values in a dictionary

Returns

An iterable over all of the valuesin d (in an unspecified order)

Raises

StopIteration when d has no more values

Examples


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Iterate over values:

forvalue in d.values():

d.items()Iterate over the elements ((key, value) pairs) in a dictionary

ReturnsAn iterable over all of the (key, value) pairs in d (in an unspecified order).

Each (key, value) pairs is represented as a tuple .

Raises

StopIteration when d has no more elements

Examples

Iterate over items:

forkey, value in d.items():

Note that, in the example, we can use multiple assignment to assign key to

the key and value to the value of each item directly in the for loop.

d.pop(key[, default])Used to remove an item from a dictionary and return its associated value

Returns

d[key] if key is in d . If key is not in d but default is specified, the

default value is returned instead.

Raises

KeyError if key is not in dictionary and no default is specified

ExamplesDelete entry with key 'hello' and print its value


hello_value =my_dictionary.pop('hello')

print(hello_value)

# 1


# {'goodbye': 2}


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With default specified


foo_value =my_dictionary.pop('foo', None)

print(foo_value)

# None


# {'goodbye': 2}

With no default specified


foo_value =my_dictionary.pop('foo')

# KeyError: 'foo'

d.popitem()Pop (i.e. delete and return) a random element from the dictionary

Returns

A (key, value) tuple if d is not empty.

Raises

KeyError if d is empty. I personally think that's a stupid exception to raise

since no key was ever specified, but, hey, I didn't write the language.

ExamplesDestructively iterate over values:

try:

key,value =d.popitem():

print 'Got {}: {}'.format(key,value)

except KeyError:

print 'Done'

d.setdefault(key[, default])Get a key from the dictionary or, if it's not there, insert it with a default

value and return that. default , erm, defaults to None

Returns

d[key] if key is in d .

If not, do d[key] = default and thenreturn d[key] (which will always

return default ).


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Raises

N/A

Examples

Count the number of times each word is seen in a file:

words ={}

forword in file:

occurrences =words.setdefault(word, 0)

words[word] =occurrences +1

d.update(other)Update a dictionary with the keys and values in other , overwriting existing

keys and values if there is any overlap.

Returns

None

Raises

N/A

Examples

Merge two dictionaries:

first ={'a':1}

second ={'b':2}

first.update(second)

print first

# {'a': 1, 'b': 2}

print second

# {'b': 2}

Using keyword arguments for other :

first ={'a':1}

first.update(b=2, c=3)

print first

# {'a': 1, 'c': 3, 'b': 2}

Posted on Aug 30, 2015 by Jeff Knupp

Previous Post: Flask and SQLAlchemy Magic
https://jeffknupp.com/blog/2015/07/12/flask-and-sqlalchemy-magic


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