Python Dictionary Examples

Dictionary. A dictionary optimizes element lookups.

It associates keys to values. Each key must have a value. Dictionaries are used in many programs.

With square brackets, we assign and access a value at a key. With get() we can specify a default result. Dictionaries are fast. We create, mutate and test them.

Get example. There are many ways to get values. We can use the "[" and "]" characters. We access a value directly this way. But this syntax causes a KeyError if the key is not found.

Instead: We can use the get() method with one or two arguments. This does not cause any annoying errors. It returns None.

Argument 1: The first argument to get() is the key you are testing. This argument is required.

Argument 2: The second, optional argument to get() is the default value. This is returned if the key is not found.

Based on:

Python 3

Python program that gets values

plants = {}

# Add three key-value tuples to the dictionary.
plants["radish"] = 2
plants["squash"] = 4
plants["carrot"] = 7

# Get syntax 1.
print(plants["radish"])

# Get syntax 2.
print(plants.get("tuna"))
print(plants.get("tuna", "no tuna found"))

Output

2
None
no tuna found

Get, none. In Python "None" is a special value like null or nil. I like None. It is my friend. It means no value. Get() returns None if no value is found in a dictionary.

None

Note: It is valid to assign a key to None. So get() can return None, but there is actually a None value in the dictionary.

Key error. Errors in programs are not there just to torment you. They indicate problems with a program and help it work better. A KeyError occurs on an invalid access.

KeyError

Python program that causes KeyError

lookup = {"cat": 1, "dog": 2}

# The dictionary has no fish key!
print(lookup["fish"])

Output

Traceback (most recent call last):
  File "C:\programs\file.py", line 5, in <module>
    print(lookup["fish"])
KeyError: 'fish'

In-keyword. A dictionary may (or may not) contain a specific key. Often we need to test for existence. One way to do so is with the in-keyword.

In

True: This keyword returns 1 (meaning true) if the key exists as part of a key-value tuple in the dictionary.

False: If the key does not exist, the in-keyword returns 0, indicating false. This is helpful in if-statements.

Python program that uses in

animals = {}
animals["monkey"] = 1
animals["tuna"] = 2
animals["giraffe"] = 4

# Use in.
if "tuna" in animals:
    print("Has tuna")
else:
    print("No tuna")

# Use in on nonexistent key.
if "elephant" in animals:
    print("Has elephant")
else:
    print("No elephant")

Output

Has tuna
No elephant

Len built-in. This returns the number of key-value tuples in a dictionary. The data types of the keys and values do not matter. Len also works on lists and strings.

Caution: The length returned for a dictionary does not separately consider keys and values. Each pair adds one to the length.

Python program that uses len on dictionary

animals = {"parrot": 2, "fish": 6}

# Use len built-in on animals.
print("Length:", len(animals))

Output

Length: 2

Len notes. Let us review. Len() can be used on other data types, not just dictionaries. It acts upon a list, returning the number of elements within. It also handles tuples.

Len

Keys, values. A dictionary contains keys. It contains values. And with the keys() and values() methods, we can store these elements in lists.

Next: A dictionary of three key-value pairs is created. This dictionary could be used to store hit counts on a website's pages.

Views: We introduce two variables, named keys and values. These are not lists—but we can convert them to lists.

Convert

Python program that uses keys

hits = {"home": 125, "sitemap": 27, "about": 43}
keys = hits.keys()
values = hits.values()

print("Keys:")
print(keys)
print(len(keys))

print("Values:")
print(values)
print(len(values))

Output

Keys:
dict_keys(['home', 'about', 'sitemap'])
3
Values:
dict_values([125, 43, 27])
3

Keys, values ordering. Elements returned by keys() and values() are not ordered. In the above output, the keys-view is not alphabetically sorted. Consider a sorted view (keep reading).

Sorted keys. In a dictionary keys are not sorted in any way. They are unordered. Their order reflects the internals of the hashing algorithm's buckets.

But: Sometimes we need to sort keys. We invoke another method, sorted(), on the keys. This creates a sorted view.

Python program that sorts keys in dictionary

# Same as previous program.
hits = {"home": 124, "sitemap": 26, "about": 32}

# Sort the keys from the dictionary.
keys = sorted(hits.keys())

print(keys)

Output

['about', 'home', 'sitemap']

Items. With this method we receive a list of two-element tuples. Each tuple contains, as its first element, the key. Its second element is the value.

Tip: With tuples, we can address the first element with an index of 0. The second element has an index of 1.

Program: The code uses a for-loop on the items() list. It uses the print() method with two arguments.

Python that uses items method

rents = {"apartment": 1000, "house": 1300}

# Convert to list of tuples.
rentItems = rents.items()

# Loop and display tuple items.
for rentItem in rentItems:
    print("Place:", rentItem[0])
    print("Cost:", rentItem[1])
    print("")

Output

Place: house
Cost: 1300

Place: apartment
Cost: 1000

Items, assign. We cannot assign elements in the tuples. If you try to assign rentItem[0] or rentItem[1], you will get an error. This is the error message.

Python error:

TypeError: 'tuple' object does not support item assignment

Items, unpack. The items() list can be used in another for-loop syntax. We can unpack the two parts of each tuple in items() directly in the for-loop.

Here: In this example, we use the identifier "k" for the key, and "v" for the value.

Python that unpacks items

# Create a dictionary.
data = {"a": 1, "b": 2, "c": 3}

# Loop over items and unpack each item.
for k, v in data.items():
    # Display key and value.
    print(k, v)

Output

a 1
c 3
b 2

For-loop. A dictionary can be directly enumerated with a for-loop. This accesses only the keys in the dictionary. To get a value, we will need to look up the value.

Items: We can call the items() method to get a list of tuples. No extra hash lookups will be needed to access values.

Here: The plant variable, in the for-loop, is the key. The value is not available—we would need plants.get(plant) to access it.

Python that loops over dictionary

plants = {"radish": 2, "squash": 4, "carrot": 7}

# Loop over dictionary directly.
# ... This only accesses keys.
for plant in plants:
    print(plant)

Output

radish
carrot
squash

Del built-in. How can we remove data? We apply the del method to a dictionary entry. In this program, we initialize a dictionary with three key-value tuples.

Del

Then: We remove the tuple with key "windows". When we display the dictionary, it now contains only two key-value pairs.

Python that uses del

systems = {"mac": 1, "windows": 5, "linux": 1}

# Remove key-value at "windows" key.
del systems["windows"]

# Display dictionary.
print(systems)

Output

{'mac': 1, 'linux': 1}

Del, alternative. An alternative to using del on a dictionary is to change the key's value to a special value. This is a null object refactoring strategy.

Update. With this method we change one dictionary to have new values from a second dictionary. Update() also modifies existing values. Here we create two dictionaries.

Pets1, pets2: The pets2 dictionary has a different value for the dog key—it has the value "animal", not "canine".

Also: The pets2 dictionary contains a new key-value pair. In this pair the key is "parakeet" and the value is "bird".

Result: Existing values are replaced with new values that match. New values are added if no matches exist.

Python that uses update

# First dictionary.
pets1 = {"cat": "feline", "dog": "canine"}

# Second dictionary.
pets2 = {"dog": "animal", "parakeet": "bird"}

# Update first dictionary with second.
pets1.update(pets2)

# Display both dictionaries.
print(pets1)
print(pets2)

Output

{'parakeet': 'bird', 'dog': 'animal', 'cat': 'feline'}
{'dog': 'animal', 'parakeet': 'bird'}

Copy. This method performs a shallow copy of an entire dictionary. Every key-value tuple in the dictionary is copied. This is not just a new variable reference.

Here: We create a copy of the original dictionary. We then modify values within the copy. The original is not affected.

Python that uses copy

original = {"box": 1, "cat": 2, "apple": 5}

# Create copy of dictionary.
modified = original.copy()

# Change copy only.
modified["cat"] = 200
modified["apple"] = 9

# Original is still the same.
print(original)
print(modified)

Output

{'box': 1, 'apple': 5, 'cat': 2}
{'box': 1, 'apple': 9, 'cat': 200}

Fromkeys. This method receives a sequence of keys, such as a list. It creates a dictionary with each of those keys. We can specify a value as the second argument.

Values: If you specify the second argument to fromdict(), each key has that value in the newly-created dictionary.

Python that uses fromkeys

# A list of keys.
keys = ["bird", "plant", "fish"]

# Create dictionary from keys.
d = dict.fromkeys(keys, 5)

# Display.
print(d)

Output

{'plant': 5, 'bird': 5, 'fish': 5}

Dict. With this built-in function, we can construct a dictionary from a list of tuples. The tuples are pairs. They each have two elements, a key and a value.

Tip: This is a possible way to load a dictionary from disk. We can store (serialize) it as a list of pairs.

Python that uses dict built-in

# Create list of tuple pairs.
# ... These are key-value pairs.
pairs = [("cat", "meow"), ("dog", "bark"), ("bird", "chirp")]

# Convert list to dictionary.
lookup = dict(pairs)

# Test the dictionary.
print(lookup.get("dog"))
print(len(lookup))

Output

bark
3

Memoize. One classic optimization is called memoization. And this can be implemented easily with a dictionary. In memoization, a function (def) computes its result.

Memoize

And: Once the computation is done, it stores its result in a cache. In the cache, the argument is the key. And the result is the value.

Memoization, continued. When a memoized function is called, it first checks this cache to see if it has been, with this argument, run before.

And: If it has, it returns its cached—memoized—return value. No further computations need be done.

Note: If a function is only called once with the argument, memoization has no benefit. And with many arguments, it usually works poorly.

Get performance. I compared a loop that uses get() with one that uses both the in-keyword and a second look up. Version 2, with the "in" operator, was faster.

Version 1: This version uses a second argument to get(). It tests that against the result and then proceeds if the value was found.

Version 2: This version uses "in" and then a lookup. Twice as many lookups occur. But fewer statements are executed.

Python that benchmarks get

import time

# Input dictionary.
systems = {"mac": 1, "windows": 5, "linux": 1}

# Time 1.
print(time.time())

# Get version.
i = 0
v = 0
x = 0
while i < 10000000:
    x = systems.get("windows", -1)
    if x != -1:
        v = x
    i += 1

# Time 2.
print(time.time())

# In version.
i = 0
v = 0
while i < 10000000:
    if "windows" in systems:
        v = systems["windows"]
    i += 1

# Time 3.
print(time.time())

Output

1345819697.257
1345819701.155 (get = 3.90 s)
1345819703.453 (in  = 2.30 s)

String key performance. In another test, I compared string keys. I found that long string keys take longer to look up than short ones. Shorter keys are faster.

Dictionary String Key

Performance, loop. A dictionary can be looped over in different ways. In this benchmark we test two approaches. We access the key and value in each iteration.

Version 1: This version loops over the keys of the dictionary with a while-loop. It then does an extra lookup to get the value.

Version 2: This version instead uses a list of tuples containing the keys and values. It actually does not touch the original dictionary.

But: Version 2 has the same effect—we access the keys and values. The cost of calling items() initially is not counted here.

Python that benchmarks loops

import time

data = {"michael": 1, "james": 1, "mary": 2, "dale": 5}
items = data.items()

print(time.time())

# Version 1: get.
i = 0
while i < 10000000:
    v = 0
    for key in data:
        v = data[key]
    i += 1

print(time.time())

# Version 2: items.
i = 0
while i < 10000000:
    v = 0
    for tuple in items:
        v = tuple[1]
    i += 1

print(time.time())

Output

1345602749.41
1345602764.29 (version 1 = 14.88 s)
1345602777.68 (version 2 = 13.39 s)

Benchmark, loop results. We see above that looping over a list of tuples is faster than directly looping over a dictionary. This makes sense. With the list, no lookups are done.

Frequencies. A dictionary can be used to count frequencies. Here we introduce a string that has some repeated letters. We use get() on a dictionary to start at 0 for nonexistent values.

So: The first time a letter is found, its frequency is set to 0 + 1, then 1 + 1. Get() has a default return.

Python that counts letter frequencies

# The first three letters are repeated.
letters = "abcabcdefghi"

frequencies = {}
for c in letters:
    # If no key exists, get returns the value 0.
    # ... We then add one to increase the frequency.
    # ... So we start at 1 and progress to 2 and then 3.
    frequencies[c] = frequencies.get(c, 0) + 1

for f in frequencies.items():
    # Print the tuple pair.
    print(f)

Output

('a', 2)
('c', 2)
('b', 2)
('e', 1)
('d', 1)
('g', 1)
('f', 1)
('i', 1)
('h', 1)

A summary. A dictionary is usually implemented as a hash table. Here a special hashing algorithm translates a key (often a string) into an integer.

For a speedup, this integer is used to locate the data. This reduces search time. For programs with performance trouble, using a dictionary is often the initial path to optimization.

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