Python Dictionaries

• A dictionary is a series of pairs of things. It is a collection which is unordered, changeable and indexed.

• Each pair contains a key—"firstname", "last name" etc.—and a value—"Mansoor", "Ahmed", etc

• To pick something out of a dictionary, we specify a particular key and

  ask what value is paired with it

• Dictionaries are written with curly brackets, and they have keys and values.

• In a dictionary, each chunk is a paired key and value.

• Notice that the key is followed by a colon:

• the variable name is singular, not plural

• All the values are strings, enclosed in quotation marks, and so are all the keys

• Dictionary’s purpose is to store information that we can later lay our hands on.

• For example, if someone wants to know the address of "Mansoor Ahmed" how does he will find it?

• In a dictionary, we pick out an element by specifying its key:

• address_of_customer = customer_12345 ["address"]

• print (address_of_customer)

• The keys can be strings and numbers.

• To pick out a value, we use the number

• Values can be numbers, too :

• To pick out a value, we use the key, a string in this case

• We can mix strings and numbers any way we want

How to add a key-value pair to a dictionary

• Let us see we have dictionary

• customer_12345 = {"first name": "Mansoor", "last name": "Ahmed", "address": "H.No.3 Ibrahim Fibres Ltd Colony"}

• We can add a new pair by writing

• customer_12345["city"]= "Faisalabad"

• We can define an empty dictionary, a dictionary with no key-value pairs:

• things_to_remember = {}

• Later, we can fill the dictionary with pairs, adding one at a time

• things_to_remember[1000] ="highest number"

• thing_to_remember["last name"]= "Ahmed"

How to delete an element from dictionary:

del customer_12345["address"]

• It begins with the same keyword:

• Then comes the name of the dictionary.

• And the particular piece of information we're after is specified by the key (in the dictionary), in square brackets

• Then the assignment of the new value.

Looping through a values

1. customer_12345 = {"first name": "Mansoor", "last name": "Ahmed", "address": "H.No.3 Ibrahim Fibres Ltd Colony"}
2. for each_value in customer_12345.value():
3. print (each_valuve)

• The loop begins with the familiar for:

• Next comes a variable to store the value for each iteration.

• Next, the keyword in followed by the name of the dictionary, customer_12345

• Then a dot…

• Then the keyword values…

• then empty parentheses and colon

Looping through keys

• This is a dictionary

  customer_12345 = {"first name": "Mansoor", "last name": "Ahmed", "address": "H.No.3 Ibrahim Fibres Ltd Colony"}

• This is the loop:

  for each_key in customer_12345.keys():

  print(each_key)

• Python displays:

  first name

  last name

  address

Looping through key-value pairs

• This is the dictionary we've been working with:

  customer_12345 = {"first name": "Mansoor", "last name": "Ahmed", "address": "H.No.3 Ibrahim Fibres Ltd Colony"}

• Here's the code for looping through the dictionary and printing all the keys and values:

  for each_key,each_value in customer_12345.items():

  print("The_customer’s" + each_key + " is" + each_value)

• Following the instructions above, Python displays:

  The customer's first name is Mansoor

  The customer's last name is Ahmed

  The customer's address is H.No.3 Ibrahim Fibres Ltd Colony

Length of Dictionary

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

  Example: Print the number of items in the dictionary:

• print(len(thisdict))

How to append a new dictionary to a list of dictionaries

• To learn how many dictionaries are in the list, we measure the list's length using the keyword len, for length, we write

• new_customer_id = len(customers)

• We can create the new dictionary

  new_dictionary = {"customer id": new_customer_id,"first name new_first_name, "last name": new_last_name,"address": new_address,}

• Finally, we append this new dictionary to the list:

• customers.append(new_dictionary)

Mansoor Ahmed Chemical Engineer,Web developer