What is a sequence object?

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

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. Just like C/C++, 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 5th element​
o

Grab a range of elements

>>> x = "hello world”"
>>> print x[4:9] # grabs 5th element to between 9th element​
o wor

Grabbing backwards from the last element

>>> x = "hello world”"
>>> print x[-3] # grabs 8th element (3 elements from end)​
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 number.

More String Manipulation

>>> 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!

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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?

Additional Standard Library Functionality

  • startswith,endswith​

  • find,rfind​

  • count​

  • isalnum​

  • strip​

  • capitalize​

  • title​

  • upper, lower​

  • swapcase​

  • center​

  • ljust,rjust​

LAB 2D and 2E:

Pull down 2D: stringsexercise.py and 2E: count_words.py and follow the instructions within.

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