Dictionary

Mar 30, 2012 Dictionaries are the fundamental data structure in Python, and a key tool in any Python programmer’s arsenal. They allow O(1) lookup speed, and have been heavily optimized for memory overhead and lookup speed efficiency. Today I”m going to show you three ways of constructing a Python dictionary, as well as some additional tips and tricks. How to Create an Empty Set in Python. In this article, we show how to create an empty set in Python. First of all, what is a set in Python? A set is a collection of unique items. So, in a set, no items can be repeated. So if you want to create a list of items in Python, in which each item is unique, then you would create a set.

A dictionary is a collection which is unordered, changeable and indexed. In Python dictionaries are written with curly brackets, and they have keys and values.

Example

Create and print a dictionary:

Python dict is an inbuilt method that constructs a dictionary in python. Dictionary means a set of data that can be unordered, changeable, and which is indexed. The dict function creates a dictionary. Dictionaries are mutable and unordered collections of key-value pairs where keys must be. With Python, creating and using a dictionary is much like working with a list, except that you must now define a key and value pair. Here are the special rules for creating a key: The key must be unique. When you enter a duplicate key, the information found in the second entry wins — the.

How do I create a dictionary with keys from a list and values defaulting to (say) zero? (you can initialize the dictionary with an empty list. Python Dictionary In this section we will learn about the dictionary data type in Python. We will demonstrate how the Python dictionaries are created in Python and how remove or add items into the Python dictionary. We will also discuss a number of Python methods that can be used with Python diction.

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Accessing Items

You can access the items of a dictionary by referring to its key name, inside square brackets:

Example

Get the value of the 'model' key:

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There is also a method called get() that will give you the same result:

Example

Get the value of the 'model' key:

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Change Values

You can change the value of a specific item by referring to its key name:

Example

Change the 'year' to 2018:

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Loop Through a Dictionary

You can loop through a dictionary by using a for loop.

When looping through a dictionary, the return value are the keys of the dictionary, but there are methods to return the values as well.

Example

Print all key names in the dictionary, one by one:

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Print all values in the dictionary, one by one:

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Example

You can also use the values() function to return values of a dictionary:

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Example

Loop through both keys and values, by using the items() function:

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Check if Key Exists

To determine if a specified key is present in a dictionary use the in keyword:

Example

Check if 'model' is present in the dictionary:

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Dictionary Length

To determine how many items (key-value pairs) a dictionary has, use the len() method.

Example

Print the number of items in the dictionary:

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Adding Items

Adding an item to the dictionary is done by using a new index key and assigning a value to it:

Example

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Removing Items

There are several methods to remove items from a dictionary:

Example

The pop() method removes the item with the specified key name:

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The popitem() method removes the last inserted item (in versions before 3.7, a random item is removed instead):

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The del keyword removes the item with the specified key name:

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The del keyword can also delete the dictionary completely:

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The clear() method empties the dictionary:

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Copy a Dictionary

You cannot copy a dictionary simply by typing dict2 = dict1, because: dict2 will only be a reference to dict1, and changes made in dict1 will automatically also be made in dict2.

There are ways to make a copy, one way is to use the built-in Dictionary method copy().

Generate A Dictionary With Keys With Empty Sets Python Download

Example

Make a copy of a dictionary with the copy() method:

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Another way to make a copy is to use the built-in method dict().

Example

Make a copy of a dictionary with the dict() method:

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Nested Dictionaries

A dictionary can also contain many dictionaries, this is called nested dictionaries.

Example

Create a dictionary that contain three dictionaries:

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Or, if you want to nest three dictionaries that already exists as dictionaries:

Example

Create three dictionaries, than create one dictionary that will contain the other three dictionaries:

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The dict() Constructor

It is also possible to use the dict() constructor to make a new dictionary:

Example

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Dictionary Methods

Python has a set of built-in methods that you can use on dictionaries.

Method	Description
clear()	Removes all the elements from the dictionary
copy()	Returns a copy of the dictionary
fromkeys()	Returns a dictionary with the specified keys and value
get()	Returns the value of the specified key
items()	Returns a list containing a tuple for each key value pair
keys()	Returns a list containing the dictionary's keys
pop()	Removes the element with the specified key
popitem()	Removes the last inserted key-value pair
setdefault()	Returns the value of the specified key. If the key does not exist: insert the key, with the specified value
update()	Updates the dictionary with the specified key-value pairs
values()	Returns a list of all the values in the dictionary

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Each key is separated from its value by a colon (:), the items are separated by commas, and the whole thing is enclosed in curly braces. An empty dictionary without any items is written with just two curly braces, like this: {}.

Keys are unique within a dictionary while values may not be. The values of a dictionary can be of any type, but the keys must be of an immutable data type such as strings, numbers, or tuples.

Accessing Values in Dictionary

To access dictionary elements, you can use the familiar square brackets along with the key to obtain its value. Following is a simple example −

When the above code is executed, it produces the following result −

If we attempt to access a data item with a key, which is not part of the dictionary, we get an error as follows −

When the above code is executed, it produces the following result −

Updating Dictionary

You can update a dictionary by adding a new entry or a key-value pair, modifying an existing entry, or deleting an existing entry as shown below in the simple example −

When the above code is executed, it produces the following result −

Delete Dictionary Elements

You can either remove individual dictionary elements or clear the entire contents of a dictionary. You can also delete entire dictionary in a single operation.

To explicitly remove an entire dictionary, just use the del statement. Following is a simple example −

This produces the following result. Note that an exception is raised because after del dict dictionary does not exist any more −

Note − del() method is discussed in subsequent section.

Properties of Dictionary Keys

Dictionary values have no restrictions. They can be any arbitrary Python object, either standard objects or user-defined objects. However, same is not true for the keys.

Dictionary With Keys Python

There are two important points to remember about dictionary keys −

(a) More than one entry per key not allowed. Gta 5 key generator for pc online. Which means no duplicate key is allowed. When duplicate keys encountered during assignment, the last assignment wins. For example −

When the above code is executed, it produces the following result −

(b) Keys must be immutable. Which means you can use strings, numbers or tuples as dictionary keys but something like ['key'] is not allowed. Following is a simple example −

When the above code is executed, it produces the following result −

Built-in Dictionary Functions & Methods

Python includes the following dictionary functions −

Sr.No.	Function with Description
1	cmp(dict1, dict2) Compares elements of both dict.
2	len(dict) Gives the total length of the dictionary. This would be equal to the number of items in the dictionary.
3	str(dict) Produces a printable string representation of a dictionary
4	type(variable) Returns the type of the passed variable. If passed variable is dictionary, then it would return a dictionary type.

Python includes following dictionary methods −

Sr.No.	Methods with Description
1	dict.clear() Removes all elements of dictionary dict
2	dict.copy() Returns a shallow copy of dictionary dict
3	dict.fromkeys() Create a new dictionary with keys from seq and values set to value.
4	dict.get(key, default=None) For key key, returns value or default if key not in dictionary
5	dict.has_key(key) Returns true if key in dictionary dict, false otherwise
6	dict.items() Returns a list of dict's (key, value) tuple pairs
7	dict.keys() Returns list of dictionary dict's keys
8	dict.setdefault(key, default=None) Similar to get(), but will set dict[key]=default if key is not already in dict
9	dict.update(dict2) Adds dictionary dict2's key-values pairs to dict
10	dict.values() Returns list of dictionary dict's values