Python Dictionaries

A dictionary is a set of unordered key, value pairs. In a dictionary, the keys must be unique and they are stored in an unordered manner.

In this tutorial you will learn the basics of how to use the Python dictionary.

Creating a Dictionary:


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


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 −


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.


Check if Exists:

You can test the presence of a key using ‘in’ or ‘not in’


Restrictions on Dictionary Keys:
Almost any type of value can be used as a dictionary key in Python. As an example,  integer, float, and Boolean objects are used as keys:


However, there are a couple restrictions that dictionary keys must abide by.

First, a given key can appear in a dictionary only once. Duplicate keys are not allowed. A dictionary maps each key to a corresponding value, so it doesn’t make sense to map a particular key more than once.
You could see below that when you assign a value to an already existing dictionary key, it does not add the key a second time, but replaces the existing value:


Restrictions on Dictionary Values:
By contrast, there are no restrictions on dictionary values. Literally none at all. A dictionary value can be any type of object Python supports, including mutable types like lists and dictionaries, and user-defined objects.There is also no restriction against a particular value appearing in a dictionary multiple times.

Hope you have enjoyed the post. Keep reading


Tuple in Python

Tuples in Python are immutable sequences of random objects. Once created, the objects within them cannot be replaced or removed,and new elements cannot be added.

Tuples have a similar syntax to lists except that they are delimited by parentheses rather than square brackets.Here’s a literal tuple containing a string, a float, and an integer.
We can access the elements of a tuple by zero-based index using square brackets, and we can determine the number of elements in the tuple using the built-in len function. We can iterate over tuples using the for loop, and we can concatenate tuples using the plus operator.


Sometimes a single element tuple is required. To write this, we can’t just use a simple number in parentheses. This is because Python pauses that as an integer enclosed in the president’s controlling parentheses of a math expression. To create a single element tuple, we make use of the trailing comma separator, which we’re allowed to use when specifying literal tuples, lists, and dictionaries.

A single element with a trailing comma is passed as a single element tuple. This leaves us with the problem of how to specify an empty tuple. In actuality the answer is simple.
We just use empty parentheses. here is the demo:


In many cases, the parentheses of literal tuples may be omitted. This feature is often used when returning multiple values from a function.

Here we make a function to return the minimum and maximum values of a series, the hard work being done by the two built-in functions min and max. Returning multiple values as tuple is often used in conjunction with a wonderful feature of Python called tuple unpacking.Tuple unpacking is a destructuring operation, which allows us to unpack data structures into named references. For example, we can assign the result of our minmax function to two new references like this.


Tuple unpacking works with arbitrarily nested tuples, although not with other data structures. Should you need to create a tuple from an existing collection object such as a list, you can use the tuple constructor, here also shown for strings.


Hope you have enjoyed the post. Please share your comments

List in python

Python lists such as those returned by the string’s split method are sequences of objects. Unlike strings, lists are mutable in so far as the elements within them can be replaced or removed, and new elements can be inserted or appended.
Literal lists are delimited by square brackets, and the items within the list separated by commas. Here is a list of three numbers and a list of three strings. We can retrieve elements by using square brackets with a zero-based index, and we can replace elements by assigning to a specific element. See how lists can be heterogeneous with respect to the types of the objects. We now have a list containing a string, an integer, and another string.


It’s often useful to create an empty list, which we can do using empty square brackets. We can modify the list in other ways.Let’s add some floats to the end of the list using the append method.


See also how we’re allowed to use an additional comma after the last element, an important maintainability feature.


Slicing is a form of extended indexing which allows us to refer to portions of a list.To use it, we pass the start and stop indices of a half-open range separated by a colon as the square brackets index argument.

Here we slice three words from the list by passing the start index 1 and the stop index 4. This facility can be combined with negative indexing. For example, to take all the elements except the first and last, slice between 1 and -1. Both the start and stop indices are optional. To slice all elements from the second to the end of the list, supply only 2: as the argument to the index operator.


we can copy one list object to another list object in the following ways:


You must be aware, that all of these techniques perform a shallow copy. That is, they create a new list containing the same object references as the source list, but don’t copy the referred to objects.

List repetition:

As for strings and tuples, lists support repetition using the multiplication operator. It’s simple enough to use. Here we repeat a list containing the integers 21 and 37 four times


To find an element in a list, use the index method passing the object you’re searching for. The elements are compared for equivalence or value equality until the one you’re looking for is found and its index returned. Here we create a list W containing few words , Searching for fox using the index method returns the integer 3, which of course allows us to get ahold of that element. If you search for a value that isn’t present like unicorn, you will receive a ValueError. Another means of searching is to count matching elements using the count method. Here we count occurrences of the word the. If you just want to test for membership, you can use the in operator or for nonmembership using the not in operator.


Elements from a list can be removed using del or remove as shown below.


 Before we move on from lists, let’s look at two operations which rearrange the elements in place, reversing and sorting. A list can be reversed in place simply by calling its reverse() method. Similarly, a list can be sorted in place using the sort() method. The sort() method accepts two optional arguments, key and reverse. The latter is self-explanatory, and when set to true gives a descending sort

Hope you have enjoyed the post.Please share your comments


Format Strings in Python

We have very powerful method str.format() in python through which you can do variable substitutions and value formatting.
Let’s understand the usage of format with couple of examples.


Formatters work by putting in one or more replacement fields and placeholders defined by a pair of curly braces { } into a string and calling the str.format(). The value we want to put into the placeholders and concatenate with the string passed as parameters into the format function.

format() method takes any number of parameters. But, is divided into two types of parameters:
Positional parameters – list of parameters that can be accessed with index of parameter inside curly braces {index} ,See the example below.


Keyword parameters – list of parameters of type key=value, that can be accessed with key of parameter inside curly braces {key}
See the example :

we have another method of formatting string literals , as shown below:


You could also call functions as shown below:


Hope you have enjoyed the post,please share your comments

Working with Strings in Python

Strings in Python are identified as a continuous set of characters represented in the quotation marks.We can access subset of strings using Index [] operator.

Slicing methods:

str = ‘SQLZealots’
print(‘str = ‘, str)

#first character
print(‘str[0] = ‘, str[0])

#last character
print(‘str[-1] = ‘, str[-1])

#slicing 2nd to 5th character
print(‘str[1:5] = ‘, str[1:5])

#slicing 6th to 2nd last character
print(‘str[5:-2] = ‘, str[5:-2])

we have many built in methods with strings. I have listed couple of them and have shown the o/p of each method with an example.

string.upper()           # get all-letters in uppercase
string.lower()           # get all-letters in lowercase
string.capitalize()    # capitalize the first letter
string.title()              # capitalize the first letter of words
string.swapcase()    # converts uppercase and lowercase
string.strip()            # remove all white spaces
string.lstrip()           # removes white space from left
string.rstrip()          # removes white space from right
string.split()             # splitting words
string.split(‘,’)          # split words by comma
string.count(‘l’)       # count how many times l is in the string
string.find(‘Wo’)      # find the word Wo in the string
string.index(“Wo”)  # find the letters Wo in the string
“:”.join(string)          # add a : between every char
” “.join(string)          # add a white space between every char
len(string)                # find the length of the string


Lets write a  small program ,to  extract the Numbers only from a given string .

Code for Extract numbers

You can find the SQL version of the code from here.

Hope you enjoyed the post. Please share your comments .

See also

Introduction to Python

First Program with Python

First Program with Python:

Lets begin our first program with some of String operations.Python has a built in String class named “str” with many handy features.String literals can be enclosed by either double or single quotes, although single quotes are more commonly used. if you want to use Multi line String then you have to use triple quotes.

First Program

Lets assign three different strings to three different variables and print them.
To start with, open Python IDLE under Start –>Programs
or Type Python Under Run prompt

IDLE is an Interactive interpreter, however, you can’t execute more than one statement at a time.
If you want to execute a multi line code program, just copy and paste the below code by opening a new file (CTRL+N) and save it. Then, you can execute the code in the file with ‘key board short cut F5’. output can be seen in the IDLE as shown below:

str1='Single Quote'
str2="Double Quotes"
str3='''This is sentence has more
than one line '''

First program

If you face any error while executing the code or any doubts ,post your query in comments section , happy to assist you.