3. An Informal Introduction to Python
*************************************

In the following examples, input and output are distinguished by the
presence or absence of prompts (*>>>* and *...*): to repeat the
example, you must type everything after the prompt, when the prompt
appears; lines that do not begin with a prompt are output from the
interpreter. Note that a secondary prompt on a line by itself in an
example means you must type a blank line; this is used to end a multi-
line command.

Many of the examples in this manual, even those entered at the
interactive prompt, include comments.  Comments in Python start with
the hash character, "#", and extend to the end of the physical line.
A comment may appear at the start of a line or following whitespace or
code, but not within a string literal.  A hash character within a
string literal is just a hash character. Since comments are to clarify
code and are not interpreted by Python, they may be omitted when
typing in examples.

Some examples:

   # this is the first comment
   spam = 1  # and this is the second comment
             # ... and now a third!
   text = "# This is not a comment because it's inside quotes."


3.1. Using Python as a Calculator
=================================

Let's try some simple Python commands.  Start the interpreter and wait
for the primary prompt, ">>>".  (It shouldn't take long.)


3.1.1. Numbers
--------------

The interpreter acts as a simple calculator: you can type an
expression at it and it will write the value.  Expression syntax is
straightforward: the operators "+", "-", "*" and "/" can be used to
perform arithmetic; parentheses ("()") can be used for grouping. For
example:

   >>> 2 + 2
   4
   >>> 50 - 5*6
   20
   >>> (50 - 5*6) / 4
   5.0
   >>> 8 / 5  # division always returns a floating-point number
   1.6

The integer numbers (e.g. "2", "4", "20") have type "int", the ones
with a fractional part (e.g. "5.0", "1.6") have type "float".  We will
see more about numeric types later in the tutorial.

Division ("/") always returns a float.  To do *floor division* and get
an integer result you can use the "//" operator; to calculate the
remainder you can use "%":

   >>> 17 / 3  # classic division returns a float
   5.666666666666667
   >>>
   >>> 17 // 3  # floor division discards the fractional part
   5
   >>> 17 % 3  # the % operator returns the remainder of the division
   2
   >>> 5 * 3 + 2  # floored quotient * divisor + remainder
   17

With Python, it is possible to use the "**" operator to calculate
powers [1]:

   >>> 5 ** 2  # 5 squared
   25
   >>> 2 ** 7  # 2 to the power of 7
   128

The equal sign ("=") is used to assign a value to a variable.
Afterwards, no result is displayed before the next interactive prompt:

   >>> width = 20
   >>> height = 5 * 9
   >>> width * height
   900

If a variable is not "defined" (assigned a value), trying to use it
will give you an error:

   >>> n  # try to access an undefined variable
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   NameError: name 'n' is not defined

There is full support for floating point; operators with mixed type
operands convert the integer operand to floating point:

   >>> 4 * 3.75 - 1
   14.0

In interactive mode, the last printed expression is assigned to the
variable "_".  This means that when you are using Python as a desk
calculator, it is somewhat easier to continue calculations, for
example:

   >>> tax = 12.5 / 100
   >>> price = 100.50
   >>> price * tax
   12.5625
   >>> price + _
   113.0625
   >>> round(_, 2)
   113.06

This variable should be treated as read-only by the user.  Don't
explicitly assign a value to it --- you would create an independent
local variable with the same name masking the built-in variable with
its magic behavior.

In addition to "int" and "float", Python supports other types of
numbers, such as "Decimal" and "Fraction". Python also has built-in
support for complex numbers, and uses the "j" or "J" suffix to
indicate the imaginary part (e.g. "3+5j").


3.1.2. Text
-----------

Python can manipulate text (represented by type "str", so-called
"strings") as well as numbers.  This includes characters ""!"", words
""rabbit"", names ""Paris"", sentences ""Got your back."", etc. ""Yay!
:)"". They can be enclosed in single quotes ("'...'") or double quotes
(""..."") with the same result [2].

>>> 'spam eggs'  # single quotes
'spam eggs'
>>> "Paris rabbit got your back :)! Yay!"  # double quotes
'Paris rabbit got your back :)! Yay!'
>>> '1975'  # digits and numerals enclosed in quotes are also strings
'1975'

To quote a quote, we need to "escape" it, by preceding it with "\".
Alternatively, we can use the other type of quotation marks:

   >>> 'doesn\'t'  # use \' to escape the single quote...
   "doesn't"
   >>> "doesn't"  # ...or use double quotes instead
   "doesn't"
   >>> '"Yes," they said.'
   '"Yes," they said.'
   >>> "\"Yes,\" they said."
   '"Yes," they said.'
   >>> '"Isn\'t," they said.'
   '"Isn\'t," they said.'

In the Python shell, the string definition and output string can look
different.  The "print()" function produces a more readable output, by
omitting the enclosing quotes and by printing escaped and special
characters:

   >>> s = 'First line.\nSecond line.'  # \n means newline
   >>> s  # without print(), special characters are included in the string
   'First line.\nSecond line.'
   >>> print(s)  # with print(), special characters are interpreted, so \n produces new line
   First line.
   Second line.

If you don't want characters prefaced by "\" to be interpreted as
special characters, you can use *raw strings* by adding an "r" before
the first quote:

   >>> print('C:\some\name')  # here \n means newline!
   C:\some
   ame
   >>> print(r'C:\some\name')  # note the r before the quote
   C:\some\name

There is one subtle aspect to raw strings: a raw string may not end in
an odd number of "\" characters; see the FAQ entry for more
information and workarounds.

String literals can span multiple lines.  One way is using triple-
quotes: """"..."""" or "'''...'''".  End of lines are automatically
included in the string, but it's possible to prevent this by adding a
"\" at the end of the line.  The following example:

   print("""\
   Usage: thingy [OPTIONS]
        -h                        Display this usage message
        -H hostname               Hostname to connect to
   """)

produces the following output (note that the initial newline is not
included):

   Usage: thingy [OPTIONS]
        -h                        Display this usage message
        -H hostname               Hostname to connect to

Strings can be concatenated (glued together) with the "+" operator,
and repeated with "*":

   >>> # 3 times 'un', followed by 'ium'
   >>> 3 * 'un' + 'ium'
   'unununium'

Two or more *string literals* (i.e. the ones enclosed between quotes)
next to each other are automatically concatenated.

   >>> 'Py' 'thon'
   'Python'

This feature is particularly useful when you want to break long
strings:

   >>> text = ('Put several strings within parentheses '
   ...         'to have them joined together.')
   >>> text
   'Put several strings within parentheses to have them joined together.'

This only works with two literals though, not with variables or
expressions:

   >>> prefix = 'Py'
   >>> prefix 'thon'  # can't concatenate a variable and a string literal
     File "<stdin>", line 1
       prefix 'thon'
              ^^^^^^
   SyntaxError: invalid syntax
   >>> ('un' * 3) 'ium'
     File "<stdin>", line 1
       ('un' * 3) 'ium'
                  ^^^^^
   SyntaxError: invalid syntax

If you want to concatenate variables or a variable and a literal, use
"+":

   >>> prefix + 'thon'
   'Python'

Strings can be *indexed* (subscripted), with the first character
having index 0. There is no separate character type; a character is
simply a string of size one:

   >>> word = 'Python'
   >>> word[0]  # character in position 0
   'P'
   >>> word[5]  # character in position 5
   'n'

Indices may also be negative numbers, to start counting from the
right:

   >>> word[-1]  # last character
   'n'
   >>> word[-2]  # second-last character
   'o'
   >>> word[-6]
   'P'

Note that since -0 is the same as 0, negative indices start from -1.

In addition to indexing, *slicing* is also supported.  While indexing
is used to obtain individual characters, *slicing* allows you to
obtain a substring:

   >>> word[0:2]  # characters from position 0 (included) to 2 (excluded)
   'Py'
   >>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
   'tho'

Slice indices have useful defaults; an omitted first index defaults to
zero, an omitted second index defaults to the size of the string being
sliced.

   >>> word[:2]   # character from the beginning to position 2 (excluded)
   'Py'
   >>> word[4:]   # characters from position 4 (included) to the end
   'on'
   >>> word[-2:]  # characters from the second-last (included) to the end
   'on'

Note how the start is always included, and the end always excluded.
This makes sure that "s[:i] + s[i:]" is always equal to "s":

   >>> word[:2] + word[2:]
   'Python'
   >>> word[:4] + word[4:]
   'Python'

One way to remember how slices work is to think of the indices as
pointing *between* characters, with the left edge of the first
character numbered 0. Then the right edge of the last character of a
string of *n* characters has index *n*, for example:

    +---+---+---+---+---+---+
    | P | y | t | h | o | n |
    +---+---+---+---+---+---+
    0   1   2   3   4   5   6
   -6  -5  -4  -3  -2  -1

The first row of numbers gives the position of the indices 0...6 in
the string; the second row gives the corresponding negative indices.
The slice from *i* to *j* consists of all characters between the edges
labeled *i* and *j*, respectively.

For non-negative indices, the length of a slice is the difference of
the indices, if both are within bounds.  For example, the length of
"word[1:3]" is 2.

Attempting to use an index that is too large will result in an error:

   >>> word[42]  # the word only has 6 characters
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   IndexError: string index out of range

However, out of range slice indexes are handled gracefully when used
for slicing:

   >>> word[4:42]
   'on'
   >>> word[42:]
   ''

Python strings cannot be changed --- they are *immutable*. Therefore,
assigning to an indexed position in the string results in an error:

   >>> word[0] = 'J'
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: 'str' object does not support item assignment
   >>> word[2:] = 'py'
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: 'str' object does not support item assignment

If you need a different string, you should create a new one:

   >>> 'J' + word[1:]
   'Jython'
   >>> word[:2] + 'py'
   'Pypy'

The built-in function "len()" returns the length of a string:

   >>> s = 'supercalifragilisticexpialidocious'
   >>> len(s)
   34

See also:

  Text Sequence Type --- str
     Strings are examples of *sequence types*, and support the common
     operations supported by such types.

  String Methods
     Strings support a large number of methods for basic
     transformations and searching.

  f-strings
     String literals that have embedded expressions.

  Format String Syntax
     Information about string formatting with "str.format()".

  printf-style String Formatting
     The old formatting operations invoked when strings are the left
     operand of the "%" operator are described in more detail here.


3.1.3. Lists
------------

Python knows a number of *compound* data types, used to group together
other values.  The most versatile is the *list*, which can be written
as a list of comma-separated values (items) between square brackets.
Lists might contain items of different types, but usually the items
all have the same type.

   >>> squares = [1, 4, 9, 16, 25]
   >>> squares
   [1, 4, 9, 16, 25]

Like strings (and all other built-in *sequence* types), lists can be
indexed and sliced:

   >>> squares[0]  # indexing returns the item
   1
   >>> squares[-1]
   25
   >>> squares[-3:]  # slicing returns a new list
   [9, 16, 25]

Lists also support operations like concatenation:

   >>> squares + [36, 49, 64, 81, 100]
   [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

Unlike strings, which are *immutable*, lists are a *mutable* type,
i.e. it is possible to change their content:

   >>> cubes = [1, 8, 27, 65, 125]  # something's wrong here
   >>> 4 ** 3  # the cube of 4 is 64, not 65!
   64
   >>> cubes[3] = 64  # replace the wrong value
   >>> cubes
   [1, 8, 27, 64, 125]

You can also add new items at the end of the list, by using the
"list.append()" *method* (we will see more about methods later):

   >>> cubes.append(216)  # add the cube of 6
   >>> cubes.append(7 ** 3)  # and the cube of 7
   >>> cubes
   [1, 8, 27, 64, 125, 216, 343]

Simple assignment in Python never copies data. When you assign a list
to a variable, the variable refers to the *existing list*. Any changes
you make to the list through one variable will be seen through all
other variables that refer to it.:

   >>> rgb = ["Red", "Green", "Blue"]
   >>> rgba = rgb
   >>> id(rgb) == id(rgba)  # they reference the same object
   True
   >>> rgba.append("Alph")
   >>> rgb
   ["Red", "Green", "Blue", "Alph"]

All slice operations return a new list containing the requested
elements.  This means that the following slice returns a shallow copy
of the list:

   >>> correct_rgba = rgba[:]
   >>> correct_rgba[-1] = "Alpha"
   >>> correct_rgba
   ["Red", "Green", "Blue", "Alpha"]
   >>> rgba
   ["Red", "Green", "Blue", "Alph"]

Assignment to slices is also possible, and this can even change the
size of the list or clear it entirely:

   >>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
   >>> letters
   ['a', 'b', 'c', 'd', 'e', 'f', 'g']
   >>> # replace some values
   >>> letters[2:5] = ['C', 'D', 'E']
   >>> letters
   ['a', 'b', 'C', 'D', 'E', 'f', 'g']
   >>> # now remove them
   >>> letters[2:5] = []
   >>> letters
   ['a', 'b', 'f', 'g']
   >>> # clear the list by replacing all the elements with an empty list
   >>> letters[:] = []
   >>> letters
   []

The built-in function "len()" also applies to lists:

   >>> letters = ['a', 'b', 'c', 'd']
   >>> len(letters)
   4

It is possible to nest lists (create lists containing other lists),
for example:

   >>> a = ['a', 'b', 'c']
   >>> n = [1, 2, 3]
   >>> x = [a, n]
   >>> x
   [['a', 'b', 'c'], [1, 2, 3]]
   >>> x[0]
   ['a', 'b', 'c']
   >>> x[0][1]
   'b'


3.2. First Steps Towards Programming
====================================

Of course, we can use Python for more complicated tasks than adding
two and two together.  For instance, we can write an initial sub-
sequence of the Fibonacci series as follows:

   >>> # Fibonacci series:
   >>> # the sum of two elements defines the next
   >>> a, b = 0, 1
   >>> while a < 10:
   ...     print(a)
   ...     a, b = b, a+b
   ...
   0
   1
   1
   2
   3
   5
   8

This example introduces several new features.

* The first line contains a *multiple assignment*: the variables "a"
  and "b" simultaneously get the new values 0 and 1.  On the last line
  this is used again, demonstrating that the expressions on the right-
  hand side are all evaluated first before any of the assignments take
  place.  The right-hand side expressions are evaluated  from the left
  to the right.

* The "while" loop executes as long as the condition (here: "a < 10")
  remains true.  In Python, like in C, any non-zero integer value is
  true; zero is false.  The condition may also be a string or list
  value, in fact any sequence; anything with a non-zero length is
  true, empty sequences are false.  The test used in the example is a
  simple comparison.  The standard comparison operators are written
  the same as in C: "<" (less than), ">" (greater than), "==" (equal
  to), "<=" (less than or equal to), ">=" (greater than or equal to)
  and "!=" (not equal to).

* The *body* of the loop is *indented*: indentation is Python's way of
  grouping statements.  At the interactive prompt, you have to type a
  tab or space(s) for each indented line.  In practice you will
  prepare more complicated input for Python with a text editor; all
  decent text editors have an auto-indent facility.  When a compound
  statement is entered interactively, it must be followed by a blank
  line to indicate completion (since the parser cannot guess when you
  have typed the last line).  Note that each line within a basic block
  must be indented by the same amount.

* The "print()" function writes the value of the argument(s) it is
  given. It differs from just writing the expression you want to write
  (as we did earlier in the calculator examples) in the way it handles
  multiple arguments, floating-point quantities, and strings.  Strings
  are printed without quotes, and a space is inserted between items,
  so you can format things nicely, like this:

     >>> i = 256*256
     >>> print('The value of i is', i)
     The value of i is 65536

  The keyword argument *end* can be used to avoid the newline after
  the output, or end the output with a different string:

     >>> a, b = 0, 1
     >>> while a < 1000:
     ...     print(a, end=',')
     ...     a, b = b, a+b
     ...
     0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,

-[ Footnotes ]-

[1] Since "**" has higher precedence than "-", "-3**2" will be
    interpreted as "-(3**2)" and thus result in "-9".  To avoid this
    and get "9", you can use "(-3)**2".

[2] Unlike other languages, special characters such as "\n" have the
    same meaning with both single ("'...'") and double (""..."")
    quotes. The only difference between the two is that within single
    quotes you don't need to escape """ (but you have to escape "\'")
    and vice versa.
