Unicode, Emoji, Escape Sequences, and f-Strings

As you now understand, strings are a data type for representing textual data.

So far, you have learned how to:

1. Write str literals, such as "hello, world"
2. Convert other types of values into to str values, str(110) evaluates to "110"
3. Build strings through concatenation "COMP" + str(110), which evaluates to "COMP110"

But what, really, are strings?

In this lesson, we will deepen on our understanding of strings. We will zoom in on strings, narrowing in on individual characters and ask what really is a character?

Characters

Textual data is made up of a sequence of characters. For example, the str "hello" comprises the characters 'h', 'e', 'l', 'l', 'o'. For the computer to represent a string, it must also be able to work with characters.

The distinction between characters and letters is noteworthy. Textual data is more than letters! There are numbers, spaces, tabs, new lines, many lettering sets 爱, and even emoji 🤠. Each is a character. Strings are sequences of characters. So, what are characters?

Like an onion, every abstraction has layers. If you cut too many layers into this onion you will leave the realm of computer science and enter the fields of physics and philosophy. You might also cry. So let’s peel back only one more layer.

What a character is depends on how it is represented. On your screen, most characters present visually as a shape or space, but some do not. A single character of data can be represented in many different visual forms by changing fonts and you can accept that it’s still the same, single character of data just viewed in a different way. Icon fonts such as wingdings intentionally present characters, even letters, as icons very different from their traditional form. Changing the font back and forth between an icon font and a traditional one doesn’t change the underlying character data, only how we see it and interpret it.

Inside a computer’s digital systems, chracters are represented as coded patterns of 0s and 1s, or binary. There is a generally agreed upon “encoding” of character data in computing systems called the American Standard Code for Information Interchange abbreviated to ASCII that was designed by humans, and agreed upon, in the 1960s. As you learned, the character A has the binary code 01000001. Numerical data, such as int values, can also be represented in binary system. It so happens that the same binary pattern 01000001 can be interpretted as the int value 65. Since binary is outside our concern, the main takeaway here is every character has a corresponding int value.

Use the ord function to find a character’s int code

Python’s built-in function ord, short for “ordinal” which is the order in which characters are defined, takes a single-character string as an input parameter and returns the int representation of the character’s binary code.

>>> ord("A")
65
>>> ord("B")
66
>>> ord("Z")
90
>>> ord("a")
97
>>> ord("b")
98
>>> ord("z")
122

There are a few important observations to make in the example above.

First, notice that the codes for letters relative to one another is logical. In the English alphabet, A is followed directly by B, just as in integers 1 is followed by 2. Similarly, A’s integer ASCII code is 65 and B’s is 66. The specific numbers of either are not important, but their relationship to one another is.

>>> "Duke" < "UNC"
True
>>> "unc" > "duke"
True
>>> "duke" > "UNC"
True

The first two examples are reasonable, but the third comes as a surprise! How is “duke” greater than “UNC”? When str values are compared in Python, and most programming languages, the ord values are compared relative to one another. The ord of lowercase letters is higher than uppercase letters in ASCII. Just as when you order information alphabetically, the first letter is compared and, if the same, the following character, and so on. (This is an algorithm!) String comparisons are case-sensitive. (For case-insensitive comparisons, you would first use str’s casefold method.)

Use the chr function to convert an int to a character

Since characters and their integer codes are two sides of the same coin, you can freely go back and forth:

>>> chr(65)
'A'
>>> chr(122)
'Z'
>>> chr(ord('A'))
'A'
>>> ord(chr(65))
65

The chr function is built-in to Python, takes an int parameter, and returns the single character representation as a string.

What about foreign languages and emoji?

When ASCII was decided in the 60s, it was a technical achievement to include both lower and uppercase letters in the standard. Emoji, and much more importantly large alphabet languages such as Chinese, were not possible until later. As additional characters were added to international standards, the set of total characters possible expanded well beyond ASCII’s initial 127 character specification.

For example, try the following in the REPL:

>>> chr(129312)

Hold on to your saddles, because we’re about to go on a little adventure. This is a bit outside of the scope of your concerns in COMP110, but to make use of Emoji in our programs (which is of utmost importance) there’s just a little more to the story to reveal.

Putting a hex on large integers

The decimal system is base-10, meaning we have 10 digits ranging from 0 through 9. Notice you grew comfortable with a 0-indexing numbering scheme in elementary school! Consider the base-10 value 90. It can be interpretted in a binary, a base-2 numeral system as 01011010. Binary is base 2 and has only 2 digits: 0 and 1. It can also be represented in a hexademical, a base-16 numeral system, with 5A. Hexadecimal is base 16 and has 16 digits, 0-9 followed by A-F which correspond to the decimal values of 10-15. Computer scientists love hexadecimal because each single digit corresponds to four binary digits. Notice that in the example: 01011010, which is 8 binary digits, is equivalent to 5A.

Python has a built-in hex function for converting to its representation. The 0x in front of the hexadecimal notation can be ignored and is case insensitive.

>>> hex(90)
0x5a

When looking up the codes for emoji or characters in other languages, they will tend to be presented to you in a hex format, such as on this web site. You will notice in the code column, there is a format of U+1F920. The U+ tells you this is Unicode, a more modern international character encoding standard than ASCII. The 1F920 is a hexadecimal representation of the code for the cowboy emoji. In Python, you can use such a Unicode character in your strings as follows:

>>> print("The \U0001F920 rides a \U0001F40E!")
The 🤠 rides a 🐎!

The leading backslash begins an escape sequence, which will be discussed in depth shortly. The U is an indication that what will follow is an 8-digit hex representation of a unicode character. Then, to encode 1F920, we must add three leading 0s for padding because 8 hex digits are expected.

It is worth taking a moment to appreciate that Python is doing a proper job of treating those emoji each as an individual item in our sequence of characters.

>>> emoji: str = "\U0001F920\U0001F40E"
>>> print(emoji)
🤠🐎
>>> len(emoji)
2
>>> emoji[0]
🤠

String Escape Sequences

In the previous example, the backslashes in the string "\U0001F920\U0001F40E" are signalling something special is about to follow the backslash. In this case, what follows is a U which hints “8 hexidecimal digits encoding a single unicode character” will follow the \U “escape sequence”.

There are other escape sequences, as well. Here are some common ones:

Escape sequences in string literals begin with a backslash.

How can you use a double quote character in a string surrounded in double quotes? With the first escape sequence above! For example, the string literal "The computer said, \"Hello, world.\"" will evaluate to the characters The computer said, "Hello, world." The \" escape sequence, when evaluated, results in a single backslash character.

The last entry in the table is also interesting. If backslashes are how we begin writing an escape sequence in a string literal in Python… how do we write a single backslash? By writing two backslashes back-to-back, of course! The first backslash begins an escape sequence, the second backslash causes the sequence to evaluate to a single backslash.

f-Strings “Format” Strings

Zooming back out to thinking of strings at a high-level, by now you have used concatenation enough to recognize that concatenating strings can be a lot of work! Especially if you are concatenating non-string values in the middle of a larger string. Modern Python has a special string literal called a format string or f-string for short, that makes this much easier. Consider the following examples:

>>> course: int = 110
>>> print("I am in COMP" + str(course) + " right now!")
I am in COMP110 right now!
>>> print(f"I am in COMP{ course } right now!")
I am in COMP110 right now!

A key distinction between a regular string and an f-string is that it begins with the letter f preceeding its quotes. Notice the difference of f"Hi" and "Hi", where the former is an f string.

Inside of an f-string you can write an expression inside of curly braces and it will get substituted with the expression’s value when the string literal is evaluated. Spaces inside of the curly braces are ignored. This is especially handy if you are building a string with multiple variables being concatenated together. Consider the difference of:

>>> name: str = "Lauren"
>>> age_turning: int = 21
>>> print("Hello " + name + ", you're almost " + str(age_turning) + "!")
Hello Lauren, you're almost 21!
>>> print(f"Hello {name}, you're almost {age_turning}!")
Hello Lauren, you're almost 21!

There are other powerful things format strings can do, too, but they are outside the scope of this course. If you’d like to learn more this guide covers many useful cases. Other modern programming languages are adopting variations of this same concept which you may also hear referred to as string interpolation.