String operations

Strings are special kinds of sequences designed to hold strings. Therefore, they have a number of methods that allow you to perform useful operations on these strings. Their detailed description can be found in the Python documentation. Below is an overview of some of the most useful operations and methods for text strings:

Finding sub-strings

Unlike other sequences, the in and not in operators work not only for individual letters but for sub-strings of text. For example, 'rous' in 'carousel' will return True.

If you need to know the position of some substring, the method find finds the position in which the given substring begins or -1 if the substring is not present in the string:

'carousel'.find('rous')    # 2

'carousel'.find('ruza')    # -1

Whitespace removal

The lstrip, rsrip and strip methods remove whitespaces (spaces, newlines, tabs) at the beginning, end, and both beginning and end of the string, respectively. This is very useful, for example, when you want to break the text into sections separated by punctuation marks (usually followed by spaces). Please try the following in the console:

words = "one, two, three"

words.split(',')                                # ['one', ' two', ' three']

[word.strip() for word in words.split(',')]     # ['one', 'two', 'three']

To see the difference between lstrip, rsrip and strip, please do in console:

text = "   AAA   "    # mind the spaces at the beginning and end

text.lstrip()

text.rstrip()

text.strip()

Case-swapping

In any text, you can change all letters to lowercase using the method lower, or to uppercase using the method upper:

text = "Alice in Wonderland"

print (text.lower())   # Alice in Wonderland

print (text.upper())   # ALICE IN WONDERLAND

This is especially useful when you want to compare two strings in a case-insensitive manner:

'AAA' == 'aaa'                   # False

'AAA'.lower() == 'Aaa'.lower()   # True

A more sophisticated method for changing the case of letters are:

capitalize: convert the first letter in the entire text to uppercase, all other letters to lowercase:

"ala has a cat".capitalize()   # 'Ala has a cat'

title: converts the first letter of each word to capital:

"ala has a cat".title()        # 'Ala Has A Cat'

String formatting

Python allows you to construct strings using values stored in other variables. There are three ways of doing this: with a % operator (this is a deprecated method and not recommended), with a format method, and with special format strings. The latter method is the easiest to use, however it requires Python version 3.6 or higher. As this method is the newest one available, it will be discussed here:

Format strings are special text strings that are created by adding a character f immediately before the double or single quotation marks that open the string (no spaces between them). In such a string, the brace has a special meaning: inside it you can put any Python expression, the value of which will be inserted into the string. E.g:

height = float(input("Enter your height [cm]:")) / 100    # we convert the given height into meters
mass = float(input("Enter your weight [kg]:")) 

bmi = weight / height ** 2     # https://en.wikipedia.org/wiki/Body_mass_index

message = f"Height: {100 * height} cm, weight: {mass} kg, BMI: {bmi}."

print(message)

Please run the above example and see what message will be displayed. Then try to remove the f prefix mark and see the difference.

In the example above, the message displayed will be:

Height: 172.0cm, weight: 64.0kg, BMI: 21.63331530557058.

Please note that the BMI value is printed with too many digits. This can be remedied. Inside the brace where the Python expression is located, we can (after the expression) add a : sign and then information how the value of this expression is to be formatted. Please change the string string with the message to the following:

message = f "Height: {100 * height} cm, weight: {mass} kg, BMI: {bmi:.2f}"

.2f after the colon it means that the value of the expression (in this case the variable bmi) is to be presented as a real number (f from float) with two decimal places (.2). Below is a list of some useful format descriptors that can appear after a colon (X and Y in the table should be replaced with integers):

Format	Description
.Yf	a real number with Y digits after the decimal dot (the value must be of type float)
Xf	a real number spanning at least X characters, if necessary, spaces will be added at the beginning (the value must be of type float)
X.Yf	number taking up at least X characters with Y places after the decimal dot (the value must be of type float)
0Xf	a real number spanning at least X characters, if necessary, zeros will be added at the beginning (the value must be of type float)
0X.Yf	like X.Yf, but empty spaces will be filled with zeros
+XXf	where XX is one of X, X.Y, 0X, 0X.Y — a sign will always be added (+ for non-negative numbers)
e	real number in exponential notation (1.4e+2) (the value must be of type float)
XXXe	for numbers in exponential notations, it is possible to force the number of digits or digits after the dot in mantissa, as above
Xd	an integer number spanning at least X characters, if necessary, spaces will be added at the beginning (the value must be of type int)
0Xd	an integer number spanning at least X characters, if necessary, zeros will be added at the beginning (the value must be of type int)
x	integer number in hexadecimal notation (the value must be of type int)
o	integer number in octal notation (the value must be of type int)
b	integer number in binary notation (the value must be of type int)
<XXs	any left-aligned expression spanning at least XX characters
>XXs	any right-aligned expression spanning at least XX characters
^XXs	any centered expression spanning at least XX characters

As an example, let’s try to print a table with the 100-meter runs world records:

# Create a list of dictionaries with data
records = [
     {'date': '16.08.2009', 'champion':' Usain Bolt', 'time': 9.58},
     {'date': '20.09.2009', 'champion' : 'Tyson Gay', 'time': 9.69},
     {'date': '23.09.2012', 'champion': 'Yohan Blake', 'time': 9.69},
     {'date': '2.09.2008', 'champion': 'Asafa Powell', 'time': 9.74}
]

width = 42            # width of the whole table
print("-" * width)   # print a horizontal line
print("|  Time  |     Champion     |    Date    |") 
print("-" * width) 
for record in records: 
    print (f"| {record['time']:6.3f} | {record ['champion']:^16s} | {record ['date']:>10s} |")
print ("-" * width)

How do I put the brace “{}” in the format string?

In a format string, to insert a brace that you want to be treated as part of the text rather than the beginning of the expression, repeat it:

f"This is a brace {{2 + 2}} and this is a brace expression {{{2 + 2}}}"

In the above text, the expression marked in blue will be evaluated and the repeated braces will appear only once.

---

Published under Creative Commons Attribution-NonCommercial-ShareAlike license.