Python Dictionary Examples

Add keys and values, and then look up them up, with a dictionary. Test lookup performance.

Dictionary. Consider a language. Each word maps to a meaning. A book is a written work. A cloud is floating water. In a dictionary we map keys (words) to values (meanings).

Data lookup. Python dictionaries are maps. With square brackets, we assign and access a value at a key. With get() we can specify a default result.

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 1 or 2 arguments. This does not cause any 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.

Python program that gets values plants = {} # Add 3 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. We often use None in programs. It means no value. Get() returns None if no value is found in a dictionary.None

Program: We see a None value for "carrot." So get() can return None, but there is actually a None value in the dictionary.

Python program that shows None in dictionary lookup = {"bird": 10, "carrot": None} # A value can be none. print("GET:", lookup.get("carrot")) print("GET:", lookup.get("xyz")) Output GET: None GET: None

KeyError. Errors in programs are not there just to annoy 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.

Tip: Len() can be used on other data types. It acts upon a list, returning the number of elements within. It also handles tuples.

Len

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

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 3 key-value pairs is created. This dictionary could be used to store hit counts on a website's pages.

Views: We introduce 2 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

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 program 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 program 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 program 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 3 key-value tuples.Del

Then: We remove the tuple with key "windows." When we display the dictionary, it now contains only 2 key-value pairs.

Python program 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.

Here: We set the "windows" key in the dictionary to 0, and this can mean no windows items exist.

Python program that uses 0 value in dictionary systems = {"mac": 1, "windows": 5, "linux": 1} # Instead of del, you can use a special value like 0. systems["windows"] = 0 # Display. print(systems) Output {'mac': 1, 'windows': 0, 'linux': 1}

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 program 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 program 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 program 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.dict

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

Python program 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

Benchmark, get. 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.

Result: It is faster to use the in-operator to test the contents of a dictionary. This approach should be preferred when possible.

Python program that benchmarks get import time # Input dictionary. systems = {"mac": 1, "windows": 5, "linux": 1} print(time.time()) # Version 1: use get. v = 0 x = 0 for i in range(10000000): x = systems.get("windows", -1) if x != -1: v = x print(time.time()) # Version 2: use in. v = 0 for i in range(10000000): if "windows" in systems: v = systems["windows"] print(time.time()) Output 1478552825.0586164 1478552827.0295532 (get = 1.97 s) 1478552828.1397061 (in = 1.11 s)

Benchmark, 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 does not touch the original dictionary.

Result: Looping over a list of tuples is faster than looping over a dictionary. This makes sense—with the list, no lookups are done.

Python program that benchmarks loops import time data = {"parrot": 1, "frog": 1, "elephant": 2, "snake": 5} items = data.items() print(time.time()) # Version 1: get. for i in range(10000000): v = 0 for key in data: v = data[key] print(time.time()) # Version 2: items. for i in range(10000000): v = 0 for tuple in items: v = tuple[1] print(time.time()) Output 1478467043.8872652 1478467048.6821966 (version 1 = 4.79 s) 1478467053.2630682 (version 2 = 4.58 s)

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 program 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)

Invert keys, values. Sometimes we want to invert a dictionary—change the values to keys, and the keys to values. Complex solutions are possible. But we can do this with a simple loop.

Python program that inverts a dictionary reptiles = {"frog": 20, "snake": 8} inverted = {} # Use items loop. # ... Turn each value into a key. for key, value in reptiles.items(): inverted[value] = key print(":::ORIGINAL:::") print(reptiles) print(":::KEYS, VALUES SWAPPED:::") print(inverted) Output :::ORIGINAL::: {'frog': 20, 'snake': 8} :::KEYS, VALUES SWAPPED::: {8: 'snake', 20: 'frog'}

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 Memoize: Lower Dictionary

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.

Add order performance. When hashes collide, a linear search must be done to locate a key in the dictionary. If we add a key earlier, it is faster to locate.Dictionary Order

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

Counter. For frequencies, we can use a special Counter from the collections module. This can make counters easier to add in programs.Counter

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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Python Dictionary Examples

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