Python

Strings

Reference: Strings

What is a sequence object?

A sequence object is a container of items accessed via index. Text strings are technically sequence objects.

Sequence Object Types:

• String
• Lists
• Tuples
• Range Objects

Each of the above support built in functions and slicing.

Sequence Objects: Strings

Strings are immutable! They need to be reassigned. There are two independent types of strings:

• ASCII: Default strings for Python 2
• Unicode (UTF-8): Default strings for Python 3

To declare a string, use one of the following. There is no Pythonic way aside from keeping it as simple and clean as possible.

• 'single quotes'
• "double quotes"
• ""escaped" quotes"
• """triple quoted-multiline"""
• r""raw" strings"

String Prefixes

• u or U for Unicode​
• r or R for raw string literal (ignores escape characters)​
• b or B for byte… is ignored in Python 2 since str is bytes.​
• A ‘u’ or ‘b’ prefix may be followed by a ‘r’ prefix.​

Yes, this stuff is old school and not very Pythonic... but it can't be helped.

Difference between ASCII and Unicode (utf-8)

ASCII defines 128 characters, mapped 0-127. Unicode on the other hand defines less than 2^21 characters, mapped 0-2^21. It is worth noting that not all Unicode numbers are assigned yet. Unicode numbers 0-128 have the same meaning as ASCII values; but do not fit into one 8-bit block of memory like ASCII values do. Thus Unicode in Python 3 utilizes utf-8 which allows for the use of multiple 8-bit blocks of memory.

Byte String vs Data String

Byte Strings are simply just a sequence of bytes. In Python 2, bytes is an alias of str. In other words, they are used interchangeably. In Python 3 on the other hand, str is it's own type... utilizing Unicode (utf-8). Whereas the bytes type in Python 3 is still a bytes object in ASCII; an array of integers.

​

Python 2

>>> x = "I am a string"
>>> type(x)
<type 'str'>​

>>> x​
'I am a string'

>>> hex(ord(x[0]))​
0x49​

Python 3

>>> x = 'I am a string'.encode('ASCII')​
>>> type(x)​
<class 'bytes'>​

>>> print(hex(x[0]))​
0x49 # ASCII code for capital I​

>>> x = b'I am a string' # Doing this in Python 2 makes no difference
>>> type(x)
<class 'bytes'>

Unicode Strings

Python 2

>>> x = u"I am a unicode string"
>>> type(x)​
<type 'unicode'>​

>>> y = unicode("Look at me! I’m a unicode string.")​
>>> type(y)​
<type 'unicode'>​

# You can use ‘…’ quotes “…” quotes or “””…””” quotes​

Python 3

>>> x = 'I am a unicode string'
>>> type(x)​
<class 'str'>​
​
# class str is unicode in Python 3 natively​
# You can use ‘…’ quotes “…” quotes or “””…””” quotes​

Slicing

Slicing allows you to grab a substring of a string. Python's indexing structure starts at 0. The only exception is when grabbing an element using a negative index. There is an example further below.

Grabbing a specific element

>>> x = "hello world"​
>>> print(x[4]) # grabs 4th index
o

Grab a range of elements

>>> x = "hello world"
>>> print(x[4:9]) # grabs 4th index up until, but not including the 9th index. 
o wor

Grabbing backwards from the last element

>>> x = "hello world"
>>> print(x[-3]) # grabs the 3rd to the last ELEMENT
r

• Slicing with negative values start at -1, not 0​. So if we have a string (x = "test"... t= -4, e = -3 s = -2 t = -1)... Thus a -2 slice will grab the 's'.
• When slicing a range, you grab everything UPTO (not including) the second defined index number.

More String Manipulation

• Try the following

>>> my_string = "Hello World!"
>>> print(my_string[0]​)
H​
>>> print(my_string[0:5])​
Hello​
>>> print(my_string[6:]​)
World!​

>>> print(my_string[-6:]) # ???​
>>> print(my_string[::2]) # ???​
>>> print(my_string  * 2) # ???​
>>> print(my_string + my_string) # ???​

What did you get? Try different operations to manipulate strings!

String Special Operators

User Input

There are a few ways to capture user input in Python. The most common are raw_input() in Python 2 and input() in Python 3. It is important that you use raw_input() in Python 2... as input() has security vulnerabilities. In Python 3, input() takes user input as a string, no matter the input. On the other hand, input() in Python 2 will take user input as the type presented. This can lead to users implementing false bools and such.

Python 2

>>> name = raw_input("What is your name? ")
What is your name? <user input>
>>> name
<user input>

Python 3

>>> name = input("What is your name? ")
What is your name? <user input>
>>> name
<user input>

As you more than likely guessed, the user input is assigned to the variable name and stored as a string. From here, you can treat var name as any other regular string.

Useful String Methods

• string.upper() # Outputs string as uppercase​
• string.lower() # Outputs string as lowercase​
• len(string) # Outputs string length​
• symbol.join(string/list) # Most commonly used to join list items, outputs string.​
• string.split(symbol )# Most commonly used to split a string up into a list of items, outputs list.​

>>> my_string = "Hello World!”"
>>> a = my_string.upper()​
>>> print(a)​
'HELLO WORLD!'​
>>> a = my_string.lower()​
>>> print(a) ​
'hello world!'​
>>> a = len(my_string)​
>>> print(a)​
12​
>>> a = my_string.split(" ")​
['Hello', 'World!']​
>>> print(my_string)​
'Hello World!'​
>>> my_string = "+".join(a)​
>>> print(my_string​)
'Hello+World!'

Changing a Character

>>> my_string = "Hello World!"​
>>> my_string[1]​
'e'
>>> my_string[1] = 'E'

What's the outcome? How does replace() work?

Join

join() combines sequential string elements by a specified _str _separator. If no separator is specified join will insert white space by default.

Syntax:

str.join(sequence)

Example:

s = "-";
seq = ("a", "b", "c"); # This is sequence of strings.
print(s.join( seq ))

#Output: a-b-c

Split

split() will breakup a string and add the data to a string array using a defined separator.

Syntax:

str.split(str="", num=string.count(str))

#str - any delimeter, by default it is space.
#num - number of lines minus one.

Example:

x = 'blue,red,green'
x.split(",")

['blue', 'red', 'green']
>>>

>>> a,b,c = x.split(",")

>>> a
'blue'

>>> b 
'red'

>>> c
'green'

Additional Standard Library Functionality

• startswith, endswith​
• find, rfind​
• count​
• isalnum​
• strip​
• capitalize​
• title​
• upper, lower​
• swapcase​
• center​
• ljust, rjust​

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