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rabi 09-18-2008 06:37 AM

chapter3
 
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.

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."


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
>>> # 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

Like in C, the equal sign ("=") is used to assign a value to a
variable. The value of an assignment is not written:

>>> 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

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()
and 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 e.g. 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.


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:

>>> '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

several lines of text just as you would do in C.

Note that whitespace at the beginning of the line is
significant."

print hello

Note that newlines would still need to be embedded in the string using

; 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

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

several lines of text much as you would do in C."

print hello

would print:

This is a rather long string containing

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 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:

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

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'

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:] # All but the first two characters
'lpA'

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] # All but 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

The best 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


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 and 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 an 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'Hellou0020World !'
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'Hellou0020World !'
u'Hello World !'
>>> ur'Hellou0020World !'
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""
u'xe4xf6xfc'
>>> str(u"")
Traceback (most recent call last):
File "<stdin>", line 1, in ?
UnicodeError: ASCII encoding error: 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"".encode('utf-8')
'xc3xa4xc3xb6xc3xbc'

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('xc3xa4xc3xb6xc3xbc', 'utf-8')
u'xe4xf6xfc'


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. 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] + ['Boe!']
['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100,
'Boe!']

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:

>>> # 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]
>>> a[:0] = a # Insert (a copy of) itself at the beginning
>>> a
[123, 'bletch', 'xyzzy', 1234, 123, 'bletch', 'xyzzy', 1234]

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

>>> len(a)
8

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
>>> p[1].append('xtra') # See section 5.1
>>> 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.


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. 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 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.

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