PY0007 Python Dictionary(Dict): Update, Cmp, Len, Sort, Copy, Items, str Example
Posted by Superadmin on November 10 2018 06:01:25

Dictionaries are another example of a data structure. A dictionary is used to map or associate things you want to store the keys you need to get them. A dictionary in Python is just like a dictionary in the real world. Python Dictionary are defined into two elements Keys and Values.

In this tutorial, we are going to learn,

Syntax for Python Dictionary:

  Dict = { ' Tim': 18,  xyz,.. }

Dictionary is listed in curly brackets, inside these curly brackets, keys and values are declared. Each key is separated from its value by a colon (:) while each element is separated by commas.

Properties of Dictionary Keys

There are two important points while using dictionary keys

Python 2 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
print (Dict['Tiffany'])

Python 3 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
print((Dict['Tiffany']))

Python Dictionary Methods

Copying dictionary

You can also copy the entire dictionary to new dictionary. For example, here we have copied our original dictionary to new dictionary name "Boys" and "Girls".

Python 2 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
Boys = {'Tim': 18,'Charlie':12,'Robert':25}
Girls = {'Tiffany':22}	
studentX=Boys.copy()
studentY=Girls.copy()
print studentX
print studentY

Python 3 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
Boys = {'Tim': 18,'Charlie':12,'Robert':25}
Girls = {'Tiffany':22}	
studentX=Boys.copy()
studentY=Girls.copy()
print(studentX)
print(studentY)

Updating Dictionary

You can also update a dictionary by adding a new entry or a key-value pair to an existing entry or by deleting an existing entry. Here in the example we will add another name "Sarah" to our existing dictionary.

Python 2 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
Dict.update({"Sarah":9})
print Dict

Python 3 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
Dict.update({"Sarah":9})
print(Dict)

Delete Keys from the dictionary

Python dictionary gives you the liberty to delete any element from the dictionary list. Suppose you don't want the name Charlie in the list, so you can delete the key element by following code.

Python 2 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
del Dict ['Charlie']
print Dict

Python 3 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
del Dict ['Charlie']
print(Dict)

When you run this code, it should print the dictionary list without Charlie.

Dictionary items() Method

The items() method returns a list of tuple pairs (Keys, Value) in the dictionary.

Python 2 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
print "Students Name: %s" % Dict.items()

Python 3 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
print("Students Name: %s" % list(Dict.items()))

Check if a given key already exists in a dictionary

For a given list, you can also check whether our child dictionary exists in a main dictionary or not. Here we have two sub-dictionaries "Boys" and "Girls", now we want to check whether our dictionary Boys exist in our main "Dict" or not. For that, we use the forloop method with else if method.

Python 2 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}
Boys = {'Tim': 18,'Charlie':12,'Robert':25}
Girls = {'Tiffany':22}
for key in Dict.keys():
    if key in Boys.keys():
        print True
    else:       
        print False

Python 3 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}
Boys = {'Tim': 18,'Charlie':12,'Robert':25}
Girls = {'Tiffany':22}
for key in list(Dict.keys()):
    if key in list(Boys.keys()):
        print(True)
    else:       
        print(False)

Sorting the Dictionary

In the dictionary, you can also sort the elements. For example, if we want to print the name of the elements of our dictionary alphabetically we have to use the forloop. It will sort each element of dictionary accordingly.

Python 2 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}
Boys = {'Tim': 18,'Charlie':12,'Robert':25}
Girls = {'Tiffany':22}
Students = Dict.keys()
Students.sort()
for S in Students:
      print":".join((S,str(Dict[S])))

Python 3 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}
Boys = {'Tim': 18,'Charlie':12,'Robert':25}
Girls = {'Tiffany':22}
Students = list(Dict.keys())
Students.sort()
for S in Students:
      print(":".join((S,str(Dict[S]))))

Python Dictionary in-built Functions

Dictionary len() Method

The len() function gives the number of pairs in the dictionary.

For example,

Python 2 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
print "Length : %d" % len (Dict)

Python 3 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
print("Length : %d" % len (Dict))

When len (Dict) function is executed it gives the output at "4" as there are four elements in our dictionary

Variable Types

Python does not require to explicitly declare the reserve memory space; it happens automatically. The assign values to variable "=" equal sign are used. The code to determine the variable type is " %type (Dict)."

Python 2 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
print "variable Type: %s" %type (Dict)

Python 3 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
print("variable Type: %s" %type (Dict))

Python List cmp() Method

The compare method cmp() is used in Python to compare values and keys of two dictionaries. If method returns 0 if both dictionaries are equal, 1 if dic1 > dict2 and -1 if dict1 < dict2.

Python 2 Example

Boys = {'Tim': 18,'Charlie':12,'Robert':25}
Girls = {'Tiffany':22}	
print cmp(Girls, Boys)

Python 3 Example

cmp is not supported in Python 3

Dictionary Str(dict)

With Str() method, you can make a dictionary into a printable string format.

Python 2 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
print "printable string:%s" % str (Dict)

Python 3 Example

Dict = {'Tim': 18,'Charlie':12,'Tiffany':22,'Robert':25}	
print("printable string:%s" % str (Dict))

Here is the list of all Dictionary Methods

MethodDescriptionSyntax
copy() Copy the entire dictionary to new dictionary dict.copy()
update() Update a dictionary by adding a new entry or a key-value pair to an
existing entry or by deleting an existing entry.
Dict.update([other])
items() Returns a list of tuple pairs (Keys, Value) in the dictionary. dictionary.items()
sort() You can sort the elements dictionary.sort()
len() Gives the number of pairs in the dictionary. len(dict)
cmp() Compare values and keys of two dictionaries cmp(dict1, dict2)
Str() Make a dictionary into a printable string format Str(dict)

Summary:

Dictionaries in a programming language is a type of data-structure used to store information connected in someway. Python Dictionary are defined into two elements Keys and Values. Dictionaries do not store their information in any particular order, so you may not get your information back in the same order you entered it.