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 "/" work just like in
most other languages (for example, Pascal or C); parentheses ("()")
can be used for grouping. For example:

   >>> 2 + 2
   4
   >>> 50 - 5*6
   20
   >>> (50 - 5.0*6) / 4
   5.0
   >>> 8 / 5.0
   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.

The return type of a division ("/") operation depends on its operands.
If both operands are of type "int", *floor division* is performed and
an "int" is returned.  If either operand is a "float", classic
division is performed and a "float" is returned.  The "//" operator is
also provided for doing floor division no matter what the operands
are.  The remainder can be calculated with the "%" operator:

   >>> 17 / 3  # int / int -> int
   5
   >>> 17 / 3.0  # int / float -> float
   5.666666666666667
   >>> 17 // 3.0  # explicit floor division discards the fractional part
   5.0
   >>> 17 % 3  # the % operator returns the remainder of the division
   2
   >>> 5 * 3 + 2  # result * 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:

   >>> 3 * 3.75 / 1.5
   7.5
   >>> 7.0 / 2
   3.5

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. Strings
--------------

Besides numbers, Python can also manipulate strings, which can be
expressed in several ways.  They can be enclosed in single quotes
("'...'") or double quotes (""..."") with the same result [2].  "\"
can be used to escape quotes:

   >>> 'spam eggs'  # single quotes
   'spam eggs'
   >>> '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 interactive interpreter, the output string is enclosed in
quotes and special characters are escaped with backslashes.  While
this might sometimes look different from the input (the enclosing
quotes could change), the two strings are equivalent.  The string is
enclosed in double quotes if the string contains a single quote and no
double quotes, otherwise it is enclosed in single quotes.  The "print"
statement produces a more readable output, by omitting the enclosing
quotes and by printing escaped and special characters:

   >>> '"Isn\'t," they said.'
   '"Isn\'t," they said.'
   >>> print '"Isn\'t," they said.'
   "Isn't," they said.
   >>> s = 'First line.\nSecond line.'  # \n means newline
   >>> s  # without print, \n is included in the output
   'First line.\nSecond line.'
   >>> print s  # with print, \n produces a 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

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
     ...
   SyntaxError: invalid syntax
   >>> ('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'

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'

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'

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'
     ...
   TypeError: 'str' object does not support item assignment
   >>> word[2:] = 'py'
     ...
   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:

  Sequence Types — str, unicode, list, tuple, bytearray, buffer,
  xrange
     Strings, and the Unicode strings described in the next section,
     are examples of *sequence types*, and support the common
     operations supported by such types.

  String Methods
     Both strings and Unicode strings support a large number of
     methods for basic transformations and searching.

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

  String Formatting Operations
     The old formatting operations invoked when strings and Unicode
     strings are the left operand of the "%" operator are described in
     more detail here.


3.1.3. Unicode Strings
----------------------

Starting with Python 2.0 a new data type for storing text data is
available to the programmer: the Unicode object. It can be used to
store and manipulate Unicode data (see http://www.unicode.org/) and
integrates well with the existing string objects, providing auto-
conversions where necessary.

Unicode has the advantage of providing one ordinal for every character
in every script used in modern and ancient texts. Previously, there
were only 256 possible ordinals for script characters. Texts were
typically bound to a code page which mapped the ordinals to script
characters. This lead to very much confusion especially with respect
to internationalization (usually written as "i18n" — "'i'" + 18
characters + "'n'") of software.  Unicode solves these problems by
defining one code page for all scripts.

Creating Unicode strings in Python is just as simple as creating
normal strings:

   >>> u'Hello World !'
   u'Hello World !'

The small "'u'" in front of the quote indicates that a Unicode string
is supposed to be created. If you want to include special characters
in the string, you can do so by using the Python *Unicode-Escape*
encoding. The following example shows how:

   >>> u'Hello\u0020World !'
   u'Hello World !'

The escape sequence "\u0020" indicates to insert the Unicode character
with the ordinal value 0x0020 (the space character) at the given
position.

Other characters are interpreted by using their respective ordinal
values directly as Unicode ordinals.  If you have literal strings in
the standard Latin-1 encoding that is used in many Western countries,
you will find it convenient that the lower 256 characters of Unicode
are the same as the 256 characters of Latin-1.

For experts, there is also a raw mode just like the one for normal
strings. You have to prefix the opening quote with ‘ur’ to have Python
use the *Raw-Unicode-Escape* encoding. It will only apply the above
"\uXXXX" conversion if there is an uneven number of backslashes in
front of the small ‘u’.

   >>> ur'Hello\u0020World !'
   u'Hello World !'
   >>> ur'Hello\\u0020World !'
   u'Hello\\\\u0020World !'

The raw mode is most useful when you have to enter lots of
backslashes, as can be necessary in regular expressions.

Apart from these standard encodings, Python provides a whole set of
other ways of creating Unicode strings on the basis of a known
encoding.

The built-in function "unicode()" provides access to all registered
Unicode codecs (COders and DECoders). Some of the more well known
encodings which these codecs can convert are *Latin-1*, *ASCII*,
*UTF-8*, and *UTF-16*. The latter two are variable-length encodings
that store each Unicode character in one or more bytes. The default
encoding is normally set to ASCII, which passes through characters in
the range 0 to 127 and rejects any other characters with an error.
When a Unicode string is printed, written to a file, or converted with
"str()", conversion takes place using this default encoding.

   >>> u"abc"
   u'abc'
   >>> str(u"abc")
   'abc'
   >>> u"äöü"
   u'\xe4\xf6\xfc'
   >>> str(u"äöü")
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128)

To convert a Unicode string into an 8-bit string using a specific
encoding, Unicode objects provide an "encode()" method that takes one
argument, the name of the encoding.  Lowercase names for encodings are
preferred.

   >>> u"äöü".encode('utf-8')
   '\xc3\xa4\xc3\xb6\xc3\xbc'

If you have data in a specific encoding and want to produce a
corresponding Unicode string from it, you can use the "unicode()"
function with the encoding name as the second argument.

   >>> unicode('\xc3\xa4\xc3\xb6\xc3\xbc', 'utf-8')
   u'\xe4\xf6\xfc'


3.1.4. 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* type), 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]

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

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

Lists also supports 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
"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]

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 b < 10:
   ...     print b
   ...     a, b = b, a+b
   ...
   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: "b < 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" statement writes the value of the expression(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 expressions 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

  A trailing comma avoids the newline after the output:

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

  Note that the interpreter inserts a newline before it prints the
  next prompt if the last line was not completed.

-[ 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.
