.. _tut-informal:

**********************************

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."

.. _tut-calculator:

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.)

.. _tut-numbers:

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

   >>> # 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 (:func:`float`,

:func:`int` and :func:`long`) don't work for complex numbers --- there is no 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.

.. _tut-strings:

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.'

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.

If we make the string literal a "raw" string, however, the ``\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.

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

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.  (The :keyword:`print` statement, described

later, can be used to write strings without quotes or escapes.)

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.  Like in Icon, 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: object doesn't support item assignment

   >>> word[:1] = 'Splat'

   Traceback (most recent call last):

     File "<stdin>", line 1, in ?

   TypeError: object doesn't 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 :func:`len` returns the length of a string::

   >>> s = 'supercalifragilisticexpialidocious'

   >>> len(s)

   34

.. seealso::

   :ref:`typesseq`

      Strings, and the Unicode strings described in the next section, are

      examples of *sequence types*, and support the common operations supported

      by such types.

   :ref:`string-methods`

      Both strings and Unicode strings support a large number of methods for

      basic transformations and searching.

   :ref:`new-string-formatting`

      Information about string formatting with :meth:`str.format` is described

      here.

   :ref:`string-formatting`

      The old formatting operations invoked when strings and Unicode strings are

      the left operand of the ``%`` operator are described in more detail here.

.. _tut-unicodestrings:

Unicode Strings

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

.. sectionauthor:: Marc-Andre Lemburg <mal@lemburg.com>

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.

.. index:: builtin: unicode

The built-in function :func:`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

:func:`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 :func:`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 :func:`unicode` function with the

encoding name as the second argument. ::

   >>> unicode('\xc3\xa4\xc3\xb6\xc3\xbc', 'utf-8')

   u'\xe4\xf6\xfc'

.. _tut-lists:

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