
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!
   STRING = "# This is not a comment."


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.)


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
   >>> # This is a comment
   ... 2+2
   4
   >>> 2+2  # and a comment on the same line as code
   4
   >>> (50-5*6)/4
   5.0
   >>> 8/5 # Fractions aren't lost when dividing integers
   1.6

Note: You might not see exactly the same result; floating point
results can differ from one machine to another.  We will say more
later about controlling the appearance of floating point output.  See
also *Floating Point Arithmetic:  Issues and Limitations* for a full
discussion of some of the subtleties of floating point numbers and
their representations.

To do integer division and get an integer result, discarding any
fractional result, there is another operator, ``//``:

   >>> # Integer division returns the floor:
   ... 7//3
   2
   >>> 7//-3
   -3

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

A value can be assigned to several variables simultaneously:

   >>> x = y = z = 0  # Zero x, y and z
   >>> x
   0
   >>> y
   0
   >>> z
   0

Variables must be "defined" (assigned a value) before they can be
used, or an error will occur:

   >>> # try to access an undefined variable
   ... n
   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

Complex numbers are also supported; imaginary numbers are written with
a suffix of ``j`` or ``J``.  Complex numbers with a nonzero real
component are written as ``(real+imagj)``, or can be created with the
``complex(real, imag)`` function.

   >>> 1j * 1J
   (-1+0j)
   >>> 1j * complex(0, 1)
   (-1+0j)
   >>> 3+1j*3
   (3+3j)
   >>> (3+1j)*3
   (9+3j)
   >>> (1+2j)/(1+1j)
   (1.5+0.5j)

Complex numbers are always represented as two floating point numbers,
the real and imaginary part.  To extract these parts from a complex
number *z*, use ``z.real`` and ``z.imag``.

   >>> a=1.5+0.5j
   >>> a.real
   1.5
   >>> a.imag
   0.5

The conversion functions to floating point and integer (``float()``,
``int()``) don't work for complex numbers --- there is not one correct
way to convert a complex number to a real number.  Use ``abs(z)`` to
get its magnitude (as a float) or ``z.real`` to get its real part:

   >>> a=3.0+4.0j
   >>> float(a)
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   TypeError: can't convert complex to float; use abs(z)
   >>> a.real
   3.0
   >>> a.imag
   4.0
   >>> abs(a)  # sqrt(a.real**2 + a.imag**2)
   5.0

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.


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:

   >>> 'spam eggs'
   'spam eggs'
   >>> 'doesn\'t'
   "doesn't"
   >>> "doesn't"
   "doesn't"
   >>> '"Yes," he said.'
   '"Yes," he said.'
   >>> "\"Yes,\" he said."
   '"Yes," he said.'
   >>> '"Isn\'t," she said.'
   '"Isn\'t," she said.'

The interpreter prints the result of string operations in the same way
as they are typed for input: inside quotes, and with quotes and other
funny characters escaped by backslashes, to show the precise value.
The string is enclosed in double quotes if the string contains a
single quote and no double quotes, else it's enclosed in single
quotes.  Once again, the ``print()`` function produces the more
readable output.

String literals can span multiple lines in several ways.  Continuation
lines can be used, with a backslash as the last character on the line
indicating that the next line is a logical continuation of the line:

   hello = "This is a rather long string containing\n\
   several lines of text just as you would do in C.\n\
       Note that whitespace at the beginning of the line is\
    significant."

   print(hello)

Note that newlines still need to be embedded in the string using
``\n``; the newline following the trailing backslash is discarded.
This example would print the following:

   This is a rather long string containing
   several lines of text just as you would do in C.
       Note that whitespace at the beginning of the line is significant.

Or, strings can be surrounded in a pair of matching triple-quotes:
``"""`` or ``'''``.  End of lines do not need to be escaped when using
triple-quotes, but they will be included in the string.

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

produces the following output:

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

If we make the string literal a "raw" string, ``\n`` sequences are not
converted to newlines, but the backslash at the end of the line, and
the newline character in the source, are both included in the string
as data.  Thus, the example:

   hello = r"This is a rather long string containing\n\
   several lines of text much as you would do in C."

   print(hello)

would print:

   This is a rather long string containing\n\
   several lines of text much as you would do in C.

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

   >>> word = 'Help' + 'A'
   >>> word
   'HelpA'
   >>> '<' + word*5 + '>'
   '<HelpAHelpAHelpAHelpAHelpA>'

Two string literals next to each other are automatically concatenated;
the first line above could also have been written ``word = 'Help'
'A'``; this only works with two literals, not with arbitrary string
expressions:

   >>> 'str' 'ing'                   #  <-  This is ok
   'string'
   >>> 'str'.strip() + 'ing'   #  <-  This is ok
   'string'
   >>> 'str'.strip() 'ing'     #  <-  This is invalid
     File "<stdin>", line 1, in ?
       'str'.strip() 'ing'
                         ^
   SyntaxError: invalid syntax

Strings can be subscripted (indexed); like in C, the first character
of a string has subscript (index) 0.  There is no separate character
type; a character is simply a string of size one.  As in the Icon
programming language, substrings can be specified with the *slice
notation*: two indices separated by a colon.

   >>> word[4]
   'A'
   >>> word[0:2]
   'He'
   >>> word[2:4]
   'lp'

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]    # The first two characters
   'He'
   >>> word[2:]    # Everything except the first two characters
   'lpA'

Unlike a C string, Python strings cannot be changed.  Assigning to an
indexed position in the string results in an error:

   >>> word[0] = 'x'
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   TypeError: 'str' object does not support item assignment
   >>> word[:1] = 'Splat'
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   TypeError: 'str' object does not support slice assignment

However, creating a new string with the combined content is easy and
efficient:

   >>> 'x' + word[1:]
   'xelpA'
   >>> 'Splat' + word[4]
   'SplatA'

Here's a useful invariant of slice operations: ``s[:i] + s[i:]``
equals ``s``.

   >>> word[:2] + word[2:]
   'HelpA'
   >>> word[:3] + word[3:]
   'HelpA'

Degenerate slice indices are handled gracefully: an index that is too
large is replaced by the string size, an upper bound smaller than the
lower bound returns an empty string.

   >>> word[1:100]
   'elpA'
   >>> word[10:]
   ''
   >>> word[2:1]
   ''

Indices may be negative numbers, to start counting from the right. For
example:

   >>> word[-1]     # The last character
   'A'
   >>> word[-2]     # The last-but-one character
   'p'
   >>> word[-2:]    # The last two characters
   'pA'
   >>> word[:-2]    # Everything except the last two characters
   'Hel'

But note that -0 is really the same as 0, so it does not count from
the right!

   >>> word[-0]     # (since -0 equals 0)
   'H'

Out-of-range negative slice indices are truncated, but don't try this
for single-element (non-slice) indices:

   >>> word[-100:]
   'HelpA'
   >>> word[-10]    # error
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   IndexError: string index out of range

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:

    +---+---+---+---+---+
    | H | e | l | p | A |
    +---+---+---+---+---+
    0   1   2   3   4   5
   -5  -4  -3  -2  -1

The first row of numbers gives the position of the indices 0...5 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.

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

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

See also:

   *Sequence Types --- str, bytes, bytearray, list, tuple, range*
      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.

   *String Formatting*
      Information about string formatting with ``str.format()`` is
      described here.

   *Old 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.


About Unicode
-------------

Starting with Python 3.0 all strings support Unicode (see
http://www.unicode.org/).

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.

If you want to include special characters in a string, you can do so
by using the Python *Unicode-Escape* encoding. The following example
shows how:

   >>> 'Hello\u0020World !'
   '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.

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

To convert a string into a sequence of bytes using a specific
encoding, string objects provide an ``encode()`` method that takes one
argument, the name of the encoding.  Lowercase names for encodings are
preferred.

   >>> "Äpfel".encode('utf-8')
   b'\xc3\x84pfel'


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.
List items need not all have the same type.

   >>> a = ['spam', 'eggs', 100, 1234]
   >>> a
   ['spam', 'eggs', 100, 1234]

Like string indices, list indices start at 0, and lists can be sliced,
concatenated and so on:

   >>> a[0]
   'spam'
   >>> a[3]
   1234
   >>> a[-2]
   100
   >>> a[1:-1]
   ['eggs', 100]
   >>> a[:2] + ['bacon', 2*2]
   ['spam', 'eggs', 'bacon', 4]
   >>> 3*a[:3] + ['Boo!']
   ['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100, 'Boo!']

Unlike strings, which are *immutable*, it is possible to change
individual elements of a list:

   >>> a
   ['spam', 'eggs', 100, 1234]
   >>> a[2] = a[2] + 23
   >>> a
   ['spam', 'eggs', 123, 1234]

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

   >>> # Replace some items:
   ... a[0:2] = [1, 12]
   >>> a
   [1, 12, 123, 1234]
   >>> # Remove some:
   ... a[0:2] = []
   >>> a
   [123, 1234]
   >>> # Insert some:
   ... a[1:1] = ['bletch', 'xyzzy']
   >>> a
   [123, 'bletch', 'xyzzy', 1234]
   >>> # Insert (a copy of) itself at the beginning
   >>> a[:0] = a
   >>> a
   [123, 'bletch', 'xyzzy', 1234, 123, 'bletch', 'xyzzy', 1234]
   >>> # Clear the list: replace all items with an empty list
   >>> a[:] = []
   >>> a
   []

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

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

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

   >>> q = [2, 3]
   >>> p = [1, q, 4]
   >>> len(p)
   3
   >>> p[1]
   [2, 3]
   >>> p[1][0]
   2

You can add something to the end of the list:

   >>> p[1].append('xtra')
   >>> p
   [1, [2, 3, 'xtra'], 4]
   >>> q
   [2, 3, 'xtra']

Note that in the last example, ``p[1]`` and ``q`` really refer to the
same object!  We'll come back to *object semantics* later.


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.  Python does not (yet!) provide an intelligent
  input line editing facility, so 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; most 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 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, 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 *end* can be used to avoid the newline after the output,
  or end the output with a different string:

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