String Operations

In the first lesson you learnt about String data type. A String data type holds text value. Value can be enclosed in a single, double or triple quote. Triple single or double quote strings are mostly used to represent strings that span multiple lines.

double_quote_word = "Python"
print(double_quote_word)

single_quote_word = 'Python'
print(single_quote_word)

multi_line_word = '''First line
  Second line''' # any white spaces are maintained 
print(multi_line_word)

multi_line_word = """First line
  Second line""" # same as the last example
print(multi_line_word)

There are many operations that you can invoke on a String object.

Here is the list of some of the operations used on a string variable;

word = 'Python'

String concatenation

Two strings can be added together with a plus (+) operator.

a = 'abc' + "def"
print(a)

Output:
abcdef

However you cannot add a string and any other data type.

a = 'abc' + 10 is invalid a = 'abc' + True is invalid

However you can convert other data types using the 'str' function

a = 'abc' + str(10) + str(True)
print(a)

Output:br> abc10True

String Special Cases

Sometimes, you want Python interpreter to parse certain characters in a string differently. This may be because these characters mean different to the parser. Here are some examples

Single or Double Quotes part of String

You want to add a single or double quote within a String literal

a = 'I'm having fun!'

In the above String literal since there is a single quote used to represent I'm the interpreter will throw an error while evaluating the String literal as it will parse the String literal upto the second single quote and thinks the rest of the String is in error. You can fix this in two ways

• Enclose the String within double quotes instead of single quotes. This can be applied to the corollary as well. I.e., having a double quote within a String enclosed within single quotes.
• Using Escape Sequence - described next.

Escape Sequence (\)

Add a '\' character to single quote. By adding this backslash character, you are essentially telling the interpreter to ignore the character next to it in a traditional way and consider that as a String literal.

a = 'I\'m having fun'

This backslash is called escape sequence, because the backslash causes the subsequent character sequence to “escape” its usual meaning.

Here are more examples of applying escape sequence

a = 'foo \
bar'      # The escape here is to ignore the newline and consider the bar within the same line.
print(a)

print("a\tb")  # adds a tab between a and b
print("a\nb")  # adds a newline between a and b

Output:
foo bar

Raw String

If you do not want to escape the characters and want to represent them as-is then you use the 'r' or 'R'

Here is an example

print(r"a\tb")

Output:
a\tb

More Examples

Strings are also a type of sequence. There is a built in function str to create strings. str is also used to get textual representation of any object.

Here are a few more commonly used convenience methods on Strings.

Using the in keyword with Strings in an 'if' Conditional

When it comes to finding substrings in a string, there are indeed many ways. You can use the find method on the string object. This method returns the index position of the first substring it finds. If there was no such string, then it returns -1. You can also use startsWith or endsWith methods to find the substring in the beginning or end of the string as the function name suggests.

However, there is another cool way of finding and that is by using the 'if' statement. If supposing you want to know if 'cat' substring is present in a 'caterpillar', then you can use the below code:

if 'cat' in 'catterpillar':
    print("Yes, this works too")

When you run the above code you will notice that the if conditional is satisfied with this expression.

format function

str.format() is a string formatting method in Python3. A String formatter works by putting in one or more placeholders defined by a pair of curly braces { } into a string and calling the str.format(). The value we wish to put into the placeholders are passed as arguments into the format function.

Syntax

str.format(positional_argument, keyword_argument)

Positional_argument can be any data type or any variables.
Keyword_argument is name/value pair passed in as argument.

Here are some examples:

Note: From Python 3.6 and upwards you should use 'f-strings' where ever applicable, in which the format expression is simplified. You can use 'f' or 'F' instead of using the format function. Here are some examples:

age = 21
message = f'My age is {age}'
print(message)
print(F'My age is {age}')
data = {'first': 'Jane', 'last': 'Doe'}
message = f'{data["first"]} {data["last"]}'
print(message)

With the f-strings notation, you can use any variable declared prior, within your expressions.

Convert numerical data to String using formatter

There are instances when you would like to format a numerical values in certain ways for readability reasons. For e.g., add thousands separator with a comma. Round a decimal number to 2 digits etc.. You can use formatters to achieve these.

Syntax :

{positional_argument_if_any:conversion_code}.format(value)

Here are some examples:

Conversion Codes

'f', 'e', ',' and 'd' shown above are not the only conversion codes. Here is the complete rundown.

• s – strings
• d – decimal integers (base-10)
• f – floating point display
• c – character
• b – binary
• o – octal
• x – hexadecimal with lowercase letters after 9
• X – hexadecimal with uppercase letters after 9
• e – exponent notation

Remember for all floating point numbers, you should use 'f' and for all whole numbers, you should use 'd'. You can add the comma for thousands separator along with 'f'

Errors: A ValueError occurs when the conversion is unsuccessful