===========================
PY4E - Python for Everybody
===========================

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`PY4E <https://www.py4e.com/>`__

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Chapter 1: Introduction Chapter 2: Variables Chapter 3: Conditionals
Chapter 4: Functions Chapter 5: Iterations Chapter 6: Strings Chapter 7:
Files Chapter 8: Lists Chapter 9: Dictionaries Chapter 10: Tuples
Chapter 11: Regex Chapter 12: Networked Programs Chapter 13: Python and
Web Services Chapter 14: Python Objects Chapter 15: Python and Databases
Chapter 16: Data Vizualization

Strings
=======

A string is a sequence
----------------------

A string is a *sequence* of characters. You can access the characters
one at a time with the bracket operator:

.. code:: python

    >>> fruit = 'banana'
    >>> letter = fruit[1]

The second statement extracts the character at index position 1 from the
``fruit`` variable and assigns it to the ``letter`` variable.

The expression in brackets is called an *index*. The index indicates
which character in the sequence you want (hence the name).

But you might not get what you expect:

.. code:: python

    >>> print(letter)
    a

For most people, the first letter of “banana” is “b”, not “a”. But in
Python, the index is an offset from the beginning of the string, and the
offset of the first letter is zero.

.. code:: python

    >>> letter = fruit[0]
    >>> print(letter)
    b

So “b” is the 0th letter (“zero-th”) of “banana”, “a” is the 1th letter
(“one-th”), and “n” is the 2th (“two-th”) letter.

|String Indexes|

String Indexes

You can use any expression, including variables and operators, as an
index, but the value of the index has to be an integer. Otherwise you
get:

.. code:: python

    >>> letter = fruit[1.5]
    TypeError: string indices must be integers

Getting the length of a string using ``len``
--------------------------------------------

``len`` is a built-in function that returns the number of characters in
a string:

.. code:: python

    >>> fruit = 'banana'
    >>> len(fruit)
    6

To get the last letter of a string, you might be tempted to try
something like this:

.. code:: python

    >>> length = len(fruit)
    >>> last = fruit[length]
    IndexError: string index out of range

The reason for the ``IndexError`` is that there is no letter in “banana”
with the index 6. Since we started counting at zero, the six letters are
numbered 0 to 5. To get the last character, you have to subtract 1 from
``length``:

.. code:: python

    >>> last = fruit[length-1]
    >>> print(last)
    a

Alternatively, you can use negative indices, which count backward from
the end of the string. The expression ``fruit[-1]`` yields the last
letter, ``fruit[-2]`` yields the second to last, and so on.

Traversal through a string with a loop
--------------------------------------

A lot of computations involve processing a string one character at a
time. Often they start at the beginning, select each character in turn,
do something to it, and continue until the end. This pattern of
processing is called a *traversal*. One way to write a traversal is with
a ``while`` loop:

.. code:: python

    index = 0
    while index < len(fruit):
        letter = fruit[index]
        print(letter)
        index = index + 1

This loop traverses the string and displays each letter on a line by
itself. The loop condition is ``index < len(fruit)``, so when ``index``
is equal to the length of the string, the condition is false, and the
body of the loop is not executed. The last character accessed is the one
with the index ``len(fruit)-1``, which is the last character in the
string.

**Exercise 1: Write a ``while`` loop that starts at the last character
in the string and works its way backwards to the first character in the
string, printing each letter on a separate line, except backwards.**

Another way to write a traversal is with a ``for`` loop:

.. code:: python

    for char in fruit:
        print(char)

Each time through the loop, the next character in the string is assigned
to the variable ``char``. The loop continues until no characters are
left.

String slices
-------------

A segment of a string is called a *slice*. Selecting a slice is similar
to selecting a character:

.. code:: python

    >>> s = 'Monty Python'
    >>> print(s[0:5])
    Monty
    >>> print(s[6:12])
    Python

The operator [n:m] returns the part of the string from the “n-th”
character to the “m-th” character, including the first but excluding the
last.

If you omit the first index (before the colon), the slice starts at the
beginning of the string. If you omit the second index, the slice goes to
the end of the string:

.. code:: python

    >>> fruit = 'banana'
    >>> fruit[:3]
    'ban'
    >>> fruit[3:]
    'ana'

If the first index is greater than or equal to the second the result is
an *empty string*, represented by two quotation marks:

.. code:: python

    >>> fruit = 'banana'
    >>> fruit[3:3]
    ''

An empty string contains no characters and has length 0, but other than
that, it is the same as any other string.

**Exercise 2: Given that ``fruit`` is a string, what does ``fruit[:]``
mean?**

Strings are immutable
---------------------

It is tempting to use the operator on the left side of an assignment,
with the intention of changing a character in a string. For example:

.. code:: python

    >>> greeting = 'Hello, world!'
    >>> greeting[0] = 'J'
    TypeError: 'str' object does not support item assignment

The “object” in this case is the string and the “item” is the character
you tried to assign. For now, an *object* is the same thing as a value,
but we will refine that definition later. An *item* is one of the values
in a sequence.

The reason for the error is that strings are *immutable*, which means
you can’t change an existing string. The best you can do is create a new
string that is a variation on the original:

.. code:: python

    >>> greeting = 'Hello, world!'
    >>> new_greeting = 'J' + greeting[1:]
    >>> print(new_greeting)
    Jello, world!

This example concatenates a new first letter onto a slice of
``greeting``. It has no effect on the original string.

Looping and counting
--------------------

The following program counts the number of times the letter “a” appears
in a string:

.. code:: python

    word = 'banana'
    count = 0
    for letter in word:
        if letter == 'a':
            count = count + 1
    print(count)

This program demonstrates another pattern of computation called a
*counter*. The variable ``count`` is initialized to 0 and then
incremented each time an “a” is found. When the loop exits, ``count``
contains the result: the total number of a’s.

**Exercise 3: Encapsulate this code in a function named ``count``, and
generalize it so that it accepts the string and the letter as
arguments.**

The ``in`` operator
-------------------

The word ``in`` is a boolean operator that takes two strings and returns
``True`` if the first appears as a substring in the second:

.. code:: python

    >>> 'a' in 'banana'
    True
    >>> 'seed' in 'banana'
    False

String comparison
-----------------

The comparison operators work on strings. To see if two strings are
equal:

.. code:: python

    if word == 'banana':
        print('All right, bananas.')

Other comparison operations are useful for putting words in alphabetical
order:

.. code:: python

    if word < 'banana':
        print('Your word,' + word + ', comes before banana.')
    elif word > 'banana':
        print('Your word,' + word + ', comes after banana.')
    else:
        print('All right, bananas.')

Python does not handle uppercase and lowercase letters the same way that
people do. All the uppercase letters come before all the lowercase
letters, so:

::

    Your word, Pineapple, comes before banana.

A common way to address this problem is to convert strings to a standard
format, such as all lowercase, before performing the comparison. Keep
that in mind in case you have to defend yourself against a man armed
with a Pineapple.

String methods
--------------

Strings are an example of Python *objects*. An object contains both data
(the actual string itself) and *methods*, which are effectively
functions that are built into the object and are available to any
*instance* of the object.

Python has a function called ``dir`` which lists the methods available
for an object. The ``type`` function shows the type of an object and the
``dir`` function shows the available methods.

.. code:: python

    >>> stuff = 'Hello world'
    >>> type(stuff)
    <class 'str'>
    >>> dir(stuff)
    ['capitalize', 'casefold', 'center', 'count', 'encode',
    'endswith', 'expandtabs', 'find', 'format', 'format_map',
    'index', 'isalnum', 'isalpha', 'isdecimal', 'isdigit',
    'isidentifier', 'islower', 'isnumeric', 'isprintable',
    'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower',
    'lstrip', 'maketrans', 'partition', 'replace', 'rfind',
    'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip',
    'split', 'splitlines', 'startswith', 'strip', 'swapcase',
    'title', 'translate', 'upper', 'zfill']
    >>> help(str.capitalize)
    Help on method_descriptor:

    capitalize(...)
        S.capitalize() -> str

        Return a capitalized version of S, i.e. make the first character
        have upper case and the rest lower case.
    >>>

While the ``dir`` function lists the methods, and you can use ``help``
to get some simple documentation on a method, a better source of
documentation for string methods would be
https://docs.python.org/library/stdtypes.html#string-methods.

Calling a *method* is similar to calling a function (it takes arguments
and returns a value) but the syntax is different. We call a method by
appending the method name to the variable name using the period as a
delimiter.

For example, the method ``upper`` takes a string and returns a new
string with all uppercase letters:

Instead of the function syntax ``upper(word)``, it uses the method
syntax ``word.upper()``.

.. code:: python

    >>> word = 'banana'
    >>> new_word = word.upper()
    >>> print(new_word)
    BANANA

This form of dot notation specifies the name of the method, ``upper``,
and the name of the string to apply the method to, ``word``. The empty
parentheses indicate that this method takes no argument.

A method call is called an *invocation*; in this case, we would say that
we are invoking ``upper`` on the ``word``.

For example, there is a string method named ``find`` that searches for
the position of one string within another:

.. code:: python

    >>> word = 'banana'
    >>> index = word.find('a')
    >>> print(index)
    1

In this example, we invoke ``find`` on ``word`` and pass the letter we
are looking for as a parameter.

The ``find`` method can find substrings as well as characters:

.. code:: python

    >>> word.find('na')
    2

It can take as a second argument the index where it should start:

.. code:: python

    >>> word.find('na', 3)
    4

One common task is to remove white space (spaces, tabs, or newlines)
from the beginning and end of a string using the ``strip`` method:

.. code:: python

    >>> line = '  Here we go  '
    >>> line.strip()
    'Here we go'

Some methods such as *startswith* return boolean values.

.. code:: python

    >>> line = 'Have a nice day'
    >>> line.startswith('Have')
    True
    >>> line.startswith('h')
    False

You will note that ``startswith`` requires case to match, so sometimes
we take a line and map it all to lowercase before we do any checking
using the ``lower`` method.

.. code:: python

    >>> line = 'Have a nice day'
    >>> line.startswith('h')
    False
    >>> line.lower()
    'have a nice day'
    >>> line.lower().startswith('h')
    True

In the last example, the method ``lower`` is called and then we use
``startswith`` to see if the resulting lowercase string starts with the
letter “h”. As long as we are careful with the order, we can make
multiple method calls in a single expression.

**Exercise 4: There is a string method called ``count`` that is similar
to the function in the previous exercise. Read the documentation of this
method at:**

https://docs.python.org/library/stdtypes.html#string-methods

**Write an invocation that counts the number of times the letter a
occurs in “banana”.**

Parsing strings
---------------

Often, we want to look into a string and find a substring. For example
if we were presented a series of lines formatted as follows:

``From stephen.marquard@``\ *`` uct.ac.za``*\ `` Sat Jan  5 09:14:16 2008``

and we wanted to pull out only the second half of the address (i.e.,
``uct.ac.za``) from each line, we can do this by using the ``find``
method and string slicing.

First, we will find the position of the at-sign in the string. Then we
will find the position of the first space *after* the at-sign. And then
we will use string slicing to extract the portion of the string which we
are looking for.

.. code:: python

    >>> data = 'From stephen.marquard@uct.ac.za Sat Jan  5 09:14:16 2008'
    >>> atpos = data.find('@')
    >>> print(atpos)
    21
    >>> sppos = data.find(' ',atpos)
    >>> print(sppos)
    31
    >>> host = data[atpos+1:sppos]
    >>> print(host)
    uct.ac.za
    >>>

We use a version of the ``find`` method which allows us to specify a
position in the string where we want ``find`` to start looking. When we
slice, we extract the characters from “one beyond the at-sign through up
to *but not including* the space character”.

The documentation for the ``find`` method is available at

https://docs.python.org/library/stdtypes.html#string-methods.

Format operator
---------------

The *format operator*, ``%`` allows us to construct strings, replacing
parts of the strings with the data stored in variables. When applied to
integers, ``%`` is the modulus operator. But when the first operand is a
string, ``%`` is the format operator.

The first operand is the *format string*, which contains one or more
*format sequences* that specify how the second operand is formatted. The
result is a string.

For example, the format sequence ``%d`` means that the second operand
should be formatted as an integer (“d” stands for “decimal”):

.. code:: python

    >>> camels = 42
    >>> '%d' % camels
    '42'

The result is the string ‘42’, which is not to be confused with the
integer value 42.

A format sequence can appear anywhere in the string, so you can embed a
value in a sentence:

.. code:: python

    >>> camels = 42
    >>> 'I have spotted %d camels.' % camels
    'I have spotted 42 camels.'

If there is more than one format sequence in the string, the second
argument has to be a
tuple\ `:sup:`1` <https://www.py4e.com/html3/06-strings#fn1>`__. Each
format sequence is matched with an element of the tuple, in order.

The following example uses ``%d`` to format an integer, ``%g`` to format
a floating-point number (don’t ask why), and ``%s`` to format a string:

.. code:: python

    >>> 'In %d years I have spotted %g %s.' % (3, 0.1, 'camels')
    'In 3 years I have spotted 0.1 camels.'

The number of elements in the tuple must match the number of format
sequences in the string. The types of the elements also must match the
format sequences:

.. code:: python

    >>> '%d %d %d' % (1, 2)
    TypeError: not enough arguments for format string
    >>> '%d' % 'dollars'
    TypeError: %d format: a number is required, not str

In the first example, there aren’t enough elements; in the second, the
element is the wrong type.

The format operator is powerful, but it can be difficult to use. You can
read more about it at

https://docs.python.org/library/stdtypes.html#printf-style-string-formatting.

Debugging
---------

A skill that you should cultivate as you program is always asking
yourself, “What could go wrong here?” or alternatively, “What crazy
thing might our user do to crash our (seemingly) perfect program?”

For example, look at the program which we used to demonstrate the
``while`` loop in the chapter on iteration:

.. code:: python

    while True:
        line = input('> ')
        if line[0] == '#':
            continue
        if line == 'done':
            break
        print(line)
    print('Done!')

    # Code: http://www.py4e.com/code3/copytildone2.py

Look what happens when the user enters an empty line of input:

.. code:: python

    > hello there
    hello there
    > # don't print this
    > print this!
    print this!
    >
    Traceback (most recent call last):
      File "copytildone.py", line 3, in <module>
        if line[0] == '#':
    IndexError: string index out of range

The code works fine until it is presented an empty line. Then there is
no zero-th character, so we get a traceback. There are two solutions to
this to make line three “safe” even if the line is empty.

One possibility is to simply use the ``startswith`` method which returns
``False`` if the string is empty.

.. code:: python

    if line.startswith('#'):

Another way is to safely write the ``if`` statement using the *guardian*
pattern and make sure the second logical expression is evaluated only
where there is at least one character in the string:

.. code:: python

    if len(line) > 0 and line[0] == '#':

Glossary
--------

counter
    A variable used to count something, usually initialized to zero and
    then incremented.
empty string
    A string with no characters and length 0, represented by two
    quotation marks.
format operator
    An operator, ``%``, that takes a format string and a tuple and
    generates a string that includes the elements of the tuple formatted
    as specified by the format string.
format sequence
    A sequence of characters in a format string, like ``%d``, that
    specifies how a value should be formatted.
format string
    A string, used with the format operator, that contains format
    sequences.
flag
    A boolean variable used to indicate whether a condition is true or
    false.
invocation
    A statement that calls a method.
immutable
    The property of a sequence whose items cannot be assigned.
index
    An integer value used to select an item in a sequence, such as a
    character in a string.
item
    One of the values in a sequence.
method
    A function that is associated with an object and called using dot
    notation.
object
    Something a variable can refer to. For now, you can use “object” and
    “value” interchangeably.
search
    A pattern of traversal that stops when it finds what it is looking
    for.
sequence
    An ordered set; that is, a set of values where each value is
    identified by an integer index.
slice
    A part of a string specified by a range of indices.
traverse
    To iterate through the items in a sequence, performing a similar
    operation on each.

Exercises
---------

**Exercise 5: Take the following Python code that stores a string:**

``str = 'X-DSPAM-Confidence:``\ **``0.8475``**\ ``'``

**Use ``find`` and string slicing to extract the portion of the string
after the colon character and then use the ``float`` function to convert
the extracted string into a floating point number.**

**Exercise 6: Read the documentation of the string methods at
https://docs.python.org/library/stdtypes.html#string-methods You might
want to experiment with some of them to make sure you understand how
they work. ``strip`` and ``replace`` are particularly useful.**

**The documentation uses a syntax that might be confusing. For example,
in ``find(sub[, start[, end]])``, the brackets indicate optional
arguments. So ``sub`` is required, but ``start`` is optional, and if you
include ``start``, then ``end`` is optional.**

--------------

#. A tuple is a sequence of comma-separated values inside a pair of
   parenthesis. We will cover tuples in Chapter
   10\ `↩︎ <https://www.py4e.com/html3/06-strings#fnref1>`__

--------------

If you find a mistake in this book, feel free to send me a fix using
`Github <https://github.com/csev/py4e/tree/master/book3>`__.

.. |String Indexes| image:: ./chap6_files/string.svg