Strings

In this tutorial, you're going to learn more about Python strings. So I've defined this course variable and set it to Python for beginners:

course = 'Python for Beginners'

now earlier I told you could use both single and double quotes to define a string, but there are times you have to use a specific form, otherwise you're going to run into issues.

Here's an example: imagine you wanted to change this string into Python's course for Beginners. So we want to add an apostrophe, lke this:

course = 'Python's course for Beginners'

You can immediately say this is going crazy, because our string starts in 'P' and then terminates in 'n'. All these characters that we have here after the second apostrophe (s course for Beginners), Python interpreter doesn't know what they are. So, to solve this problem, we need to use double quotes to define our string, so we can have a single quote in the middle of the string. So, let's change this to double quotes, now you can see it adds another double quote to close it. You have to manually remove this, and also one more time at the beginning of the string, we need to add another double quote.

course = "Python's course for Beginners"

Now you can see error is gone, so if you print course

print(course)

we see Python's course for Beginners.

Now let's say we don't want this apostrophe here, so we have "Python for Begginers", but we want to put Beginners in double quotes. Once again, if you add a double quote here, Python interpreter gets confused because it assumes the second double quote indicates that end of the string, so it doesn't know what these characters are:

course = "Python for "Beginners"
print(course)

So, to solve this, we need to change our double quotes to single quotes, and then we can add double quotes in the middle of the string:

course = 'Python for "Beginners"'
print(course)

So there are the cases for using single or double quotes. Now, in all the examples, I've shown you so far we only deal with short strings, but what if you wanted to define a string that is multiple lenghts? For example, what if you wanted to define a string for the message that we send in an email. In that case, we need to use triple quotes. So, we have 3 quotes to start out our string and 3 quotes to terminate it. Again, these quotes can be single or double quotes. Now, with this we can define a string that spans multiple lines. For example, we can say Hi Teresa, Here is our first email to you. Thank you, Patrick:

course = '''
Hi Teresa,

Here is our first email to you.

Thank you,

Patrick
'''
print(course)

So, we get this beautiful multi-line string. Now, let's change this back to something simple, so, we can look at other characteristics of strings in Python. I'm going to use single quotes and set the course name to Python for Beginners. Now here we're going to use square brackets to get a character and a given index in this string. So, to get the first character we use square brackets and type 0:

course = 'Python for Beginners'
print(course[0])

So the index of the first character in this string is 0. In other words, this is how Python strings are indexed: 0, 1, 2, 3, etc So, the index of the first character is 0, the index of the second is 1, and so on. So, if we run this program, we get p. We can also use a negative index here. And this is one of the features that we don't have in other programming languages as far as I know. So we have negative index, we can get the characters started from the end. So if I pass negative 1 here, assuming that 0 is the first character, negative 1 is the last character. So, when we run this program we shoul see s:

course = 'Python for Beginners'
print(course[-1])

If we pass negative 2, this will return the second character from the end (r). Okay? So, please pay close attention to this square brackets syntax, because quite often it's the topic for online Python tests or university exams. We can also use a similar sintax to extract a few characters instead of one character. For example, if we type:

course = 'Python for Beginners'
print(course[0:3])

Python interpreter will return all the characters starting with 0 all the way to 3, but it does not return the character at the index 3. Now here we also have default values for the start and end index. So, if we don't supply the end index, Python will return all the characters to the end of the string:

course = 'Python for Beginners'
print(course[0:])

But if you change the start index to 1, this will exclude the first character:

course = 'Python for Beginners'
print(course[1:])

Similarly, we have a default value for the start index. So, if we don't supply it, and add and end index like 5, Python will assume 0 as the start index:

course = 'Python for Beginners'
print(course[:5])

Now what if we leave both the start and the end index? In this case 0 will be assumed as the start index and the lenght of the string will be assumed as the end index. So with this syntax, you can basically copy or clone a string:

course = 'Python for Beginners'
print(course[:])

In other words, if I define another variable here, let's call it another and set it like this:

course = 'Python for Beginners'
another = course [:]
print(another)

The second variable will be the copy of the first. Now here's a little exercise for you: Define a variable, called name, and set it to Jane. When we print:

course = 'Jane'
print(course[1:-1])

What do you think we're going to see on the terminal?

Formatted Strings

Formatted strings are particularly useful in situations where you dynamically generate some text with your variables. Let's say we have two variables:

first_name = 'Jane'
last_name = 'Joplin'

So let's say with these two variables, we want to generate some text like this:

first_name = 'Jane'
last_name = 'Joplin'
Jane [Joplin] was a singer

How we do this? We define another variable like message, add the first name and concatenate this with a string that contains a space and a sqare bracket. Next, we need to add a last name, then we need to add a string that contains the closing square brackets followed by was a singer

first_name = 'Jane'
last_name = 'Joplin'
message = first_name + ' [' + last_name + '] was a singer'
print(message)

While this approach perfectly works, it's not ideal because as our text gets more complicated it becomes harder to visualize the output. So someone else reading this code, they have to visualize all the string concatenations in their head. This is where we use formatted strings, they make it easier for us to visualize the output.

So, let's define another variable called msg, and set this to a formatted string.

A formatted string is one that is prefixed with a f.

msg = f'{first_name} [{last_name}] was a singer'
print(msg)

With these curly braces, we're defining place holders or holes in our string, and when we run our program these holes will be filled with the value of our variables. So here we have 2 placeholders.

With this formatted string we can easily visualize what the output looks like.

So, to define formatted strings, prefix it with an f and add curly braces to dynamically insert values into your strings.

String Methods

To calculate the number of characters in the string below, we can use a built in function called len:

course = 'Python for Beginners'
print(len(course))

So, as you can see, we have 20 characters in this string. This is particularly useful when you receive input from the user. For example, you have noticed that when you fill out a form online, each input field quite often has a limit. For example, you might have 50 characters for your name, so using this lenght function we can enforce a limit on the number of characters in an input field. If the user types in moe characters than we allow, we can display an error.

Now, this len function is another function built into Python, it's a general purpose function, so it's not limited to couting the number of characters in a string. We can use this function to count the number of items in a list.

Now we also have functions specifically for strings. We refer to these functions as methods (object oriented programming style). To access these functions, we use '.' operator:

• for converting all these characters to upper case

course = 'Python for Beginners'
print(course.upper())

• for converting all to lower case

print(course.lower())

Note that these methods do not change the original strings; in fact, they create new ones and return its. So, if we print out our course variable right after we call the upper method, we can see that our course variable still has it's original form:

course = 'Python for Beginners'
print(course.upper())
print(course)

Now if you want to find a character or a sequence of characters in a string, you can use the find method. This will return the index of the first occurrence of character 'o':

course = 'Python for Beginners'
print(course.find('o'))

Note that the find method is sensitive to lower case and upper case characters. For example, if we pass the upper character 'O', it returns '-1' because we don't have an upper case in this string:

course = 'Python for Beginners'
print(course.find('O'))

We can also pass a sequence of characters:

course = 'Python for Beginners'
print(course.find('Beginners'))

We get '11', because the word starts woth index 11.

We also have a method for replacing a character or a sequence of characters: replace.

course = 'Python for Beginners'
print(course.replace('Beginners', 'Managers'))

The same method can be use to replace a single one character:

course = 'Python for Beginners'
print(course.replace('B', 'T'))

This method, like the find, is case sensitive.

There are times you want to chek the existence of a character or sequence of characters in your string. In those situations, you use the in operator. So let's say you want to know if this string contains the word 'Python':

print('Python' in course)

This is a boolean expression and get true or false. An it is another case sensitive method:

print('python' in course)