Add a key:value pair to dictionary in Python Dictionary in Python is an unordered collection of data values, used to store data values like a map, which unlike other Data Types that hold only single value as an element, Dictionary holds key:value pair. Looking up or setting a value in a dict uses square brackets, e.g. Dict'foo' looks up the value under the key 'foo'. Strings, numbers, and tuples work as keys, and any type can be a value. Other types may or may not work correctly as keys (strings and tuples work cleanly since they are immutable).
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.
Create and print a dictionary:
You can access the items of a dictionary by referring to its key name, inside square brackets:
Get the value of the 'model' key:
Try it Yourself »There is also a method called get()
that will give you the same result:
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Try it Yourself »You can change the value of a specific item by referring to its key name:
Change the 'year' to 2018:
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.
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Print all key names in the dictionary, one by one:
Try it Yourself »Print all values in the dictionary, one by one:
Try it Yourself »You can also use the values()
function to return values of a dictionary:
Loop through both keys and values, by using the items()
function:
To determine if a specified key is present in a dictionary use the in
keyword:
Check if 'model' is present in the dictionary:
To determine how many items (key-value pairs) a dictionary has, use the len()
method.
Print the number of items in the dictionary:
Try it Yourself »Adding an item to the dictionary is done by using a new index key and assigning a value to it:
There are several methods to remove items from a dictionary:
The pop()
method removes the item with the specified key name:
The popitem()
method removes the last inserted item (in versions before 3.7, a random item is removed instead):
The del
keyword removes the item with the specified key name:
The del
keyword can also delete the dictionary completely:
The clear()
method empties the 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()
.
Make a copy of a dictionary with the copy()
method:
Another way to make a copy is to use the built-in method dict()
.
Make a copy of a dictionary with the dict()
method:
A dictionary can also contain many dictionaries, this is called nested dictionaries.
Create a dictionary that contain three dictionaries:
Or, if you want to nest three dictionaries that already exists as dictionaries:
Create three dictionaries, than create one dictionary that will contain the other three dictionaries:
It is also possible to use the dict() constructor to make a new dictionary:
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 |
Python's efficient key/value hash table structure is called a 'dict'. The contents of a dict can be written as a series of key:value pairs within braces { }, e.g. dict = {key1:value1, key2:value2, .. }. The 'empty dict' is just an empty pair of curly braces {}.
Looking up or setting a value in a dict uses square brackets, e.g. dict['foo'] looks up the value under the key 'foo'. Strings, numbers, and tuples work as keys, and any type can be a value. Other types may or may not work correctly as keys (strings and tuples work cleanly since they are immutable). Looking up a value which is not in the dict throws a KeyError -- use 'in' to check if the key is in the dict, or use dict.get(key) which returns the value or None if the key is not present (or get(key, not-found) allows you to specify what value to return in the not-found case).
A for loop on a dictionary iterates over its keys by default. The keys will appear in an arbitrary order. The methods dict.keys() and dict.values() return lists of the keys or values explicitly. There's also an items() which returns a list of (key, value) tuples, which is the most efficient way to examine all the key value data in the dictionary. All of these lists can be passed to the sorted() function.
There are 'iter' variants of these methods called iterkeys(), itervalues() and iteritems() which avoid the cost of constructing the whole list -- a performance win if the data is huge. However, I generally prefer the plain keys() and values() methods with their sensible names. In Python 3000 revision, the need for the iterkeys() variants is going away.
Strategy note: from a performance point of view, the dictionary is one of your greatest tools, and you should use it where you can as an easy way to organize data. For example, you might read a log file where each line begins with an IP address, and store the data into a dict using the IP address as the key, and the list of lines where it appears as the value. Once you've read in the whole file, you can look up any IP address and instantly see its list of lines. The dictionary takes in scattered data and makes it into something coherent.
The % operator works conveniently to substitute values from a dict into a string by name:
The 'del' operator does deletions. In the simplest case, it can remove the definition of a variable, as if that variable had not been defined. Del can also be used on list elements or slices to delete that part of the list and to delete entries from a dictionary.
The open() function opens and returns a file handle that can be used to read or write a file in the usual way. The code f = open('name', 'r') opens the file into the variable f, ready for reading operations, and use f.close() when finished. Instead of 'r', use 'w' for writing, and 'a' for append. The special mode 'rU' is the 'Universal' option for text files where it's smart about converting different line-endings so they always come through as a simple 'n'. The standard for-loop works for text files, iterating through the lines of the file (this works only for text files, not binary files). The for-loop technique is a simple and efficient way to look at all the lines in a text file:
Reading one line at a time has the nice quality that not all the file needs to fit in memory at one time -- handy if you want to look at every line in a 10 gigabyte file without using 10 gigabytes of memory. The f.readlines() method reads the whole file into memory and returns its contents as a list of its lines. The f.read() method reads the whole file into a single string, which can be a handy way to deal with the text all at once, such as with regular expressions we'll see later.
For writing, f.write(string) method is the easiest way to write data to an open output file. Or you can use 'print' with an open file, but the syntax is nasty: 'print >> f, string'. In python 3000, the print syntax will be fixed to be a regular function call with a file= optional argument: 'print(string, file=f)'.
The 'codecs' module provides support for reading a unicode file.
For writing, use f.write() since print does not fully support unicode.
Building a Python program, don't write the whole thing in one step. Instead identify just a first milestone, e.g. 'well the first step is to extract the list of words.' Write the code to get to that milestone, and just print your data structures at that point, and then you can do a sys.exit(0) so the program does not run ahead into its not-done parts. Once the milestone code is working, you can work on code for the next milestone. Being able to look at the printout of your variables at one state can help you think about how you need to transform those variables to get to the next state. Python is very quick with this pattern, allowing you to make a little change and run the program to see how it works. Take advantage of that quick turnaround to build your program in little steps.
Combining all the basic Python material -- strings, lists, dicts, tuples, files -- try the summary wordcount.py exercise in the Basic Exercises.