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 <div class="section" id="an-informal-introduction-to-python">
<h1>3. An Informal Introduction to Python<a class="headerlink" href="#an-informal-introduction-to-python" title="Permalink to this headline">¶</a></h1>
In the following examples, input and output are distinguished by the presence or
absence of prompts (<tt class="docutils literal">&gt;&gt;&gt;</tt> and <tt class="docutils literal">...</tt>): 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,
<tt class="docutils literal">#</tt>, 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:
<div class="highlight-python"><div class="highlight"><pre># this is the first comment
SPAM = 1 # and this is the second comment
 # ... and now a third!
STRING = &quot;# This is not a comment.&quot;
</pre></div>
</div>
<div class="section" id="using-python-as-a-calculator">
<h2>3.1. Using Python as a Calculator<a class="headerlink" href="#using-python-as-a-calculator" title="Permalink to this headline">¶</a></h2>
Let&#8217;s try some simple Python commands. Start the interpreter and wait for the
primary prompt, <tt class="docutils literal">&gt;&gt;&gt;</tt>. (It shouldn&#8217;t take long.)
<div class="section" id="numbers">
<h3>3.1.1. Numbers<a class="headerlink" href="#numbers" title="Permalink to this headline">¶</a></h3>
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 <tt class="docutils literal">+</tt>, <tt class="docutils literal">-</tt>, <tt class="docutils literal">*</tt> and <tt class="docutils literal">/</tt> work just like in most other languages
(for example, Pascal or C); parentheses can be used for grouping. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; 2+2
4
&gt;&gt;&gt; # This is a comment
... 2+2
4
&gt;&gt;&gt; 2+2 # and a comment on the same line as code
4
&gt;&gt;&gt; (50-5*6)/4
5
&gt;&gt;&gt; # Integer division returns the floor:
... 7/3
2
&gt;&gt;&gt; 7/-3
-3
</pre></div>
</div>
The equal sign (<tt class="docutils literal">'='</tt>) is used to assign a value to a variable. Afterwards, no
result is displayed before the next interactive prompt:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; width = 20
&gt;&gt;&gt; height = 5*9
&gt;&gt;&gt; width * height
900
</pre></div>
</div>
A value can be assigned to several variables simultaneously:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; x = y = z = 0 # Zero x, y and z
&gt;&gt;&gt; x
0
&gt;&gt;&gt; y
0
&gt;&gt;&gt; z
0
</pre></div>
</div>
Variables must be &#8220;defined&#8221; (assigned a value) before they can be used, or an
error will occur:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; n # try to access an undefined variable
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
NameError: name &#39;n&#39; is not defined
</pre></div>
</div>
There is full support for floating point; operators with mixed type operands
convert the integer operand to floating point:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; 3 * 3.75 / 1.5
7.5
&gt;&gt;&gt; 7.0 / 2
3.5
</pre></div>
</div>
Complex numbers are also supported; imaginary numbers are written with a suffix
of <tt class="docutils literal">j</tt> or <tt class="docutils literal">J</tt>. Complex numbers with a nonzero real component are written as
<tt class="docutils literal">(real+imagj)</tt>, or can be created with the <tt class="docutils literal">complex(real, imag)</tt> function.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; 1j * 1J
(-1+0j)
&gt;&gt;&gt; 1j * complex(0,1)
(-1+0j)
&gt;&gt;&gt; 3+1j*3
(3+3j)
&gt;&gt;&gt; (3+1j)*3
(9+3j)
&gt;&gt;&gt; (1+2j)/(1+1j)
(1.5+0.5j)
</pre></div>
</div>
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
<tt class="docutils literal">z.real</tt> and <tt class="docutils literal">z.imag</tt>.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a=1.5+0.5j
&gt;&gt;&gt; a.real
1.5
&gt;&gt;&gt; a.imag
0.5
</pre></div>
</div>
The conversion functions to floating point and integer (<a class="reference internal" href="../library/functions.html#float" title="float"><tt class="xref py py-func docutils literal">float()</tt></a>,
<a class="reference internal" href="../library/functions.html#int" title="int"><tt class="xref py py-func docutils literal">int()</tt></a> and <a class="reference internal" href="../library/functions.html#long" title="long"><tt class="xref py py-func docutils literal">long()</tt></a>) don&#8217;t work for complex numbers &#8212; there is no one
correct way to convert a complex number to a real number. Use <tt class="docutils literal">abs(z)</tt> to get
its magnitude (as a float) or <tt class="docutils literal">z.real</tt> to get its real part.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a=3.0+4.0j
&gt;&gt;&gt; float(a)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
TypeError: can&#39;t convert complex to float; use abs(z)
&gt;&gt;&gt; a.real
3.0
&gt;&gt;&gt; a.imag
4.0
&gt;&gt;&gt; abs(a) # sqrt(a.real**2 + a.imag**2)
5.0
</pre></div>
</div>
In interactive mode, the last printed expression is assigned to the variable
<tt class="docutils literal">_</tt>. This means that when you are using Python as a desk calculator, it is
somewhat easier to continue calculations, for example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; tax = 12.5 / 100
&gt;&gt;&gt; price = 100.50
&gt;&gt;&gt; price * tax
12.5625
&gt;&gt;&gt; price + _
113.0625
&gt;&gt;&gt; round(_, 2)
113.06
</pre></div>
</div>
This variable should be treated as read-only by the user. Don&#8217;t explicitly
assign a value to it &#8212; you would create an independent local variable with the
same name masking the built-in variable with its magic behavior.
</div>
<div class="section" id="strings">
<h3>3.1.2. Strings<a class="headerlink" href="#strings" title="Permalink to this headline">¶</a></h3>
Besides numbers, Python can also manipulate strings, which can be expressed in
several ways. They can be enclosed in single quotes or double quotes:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; &#39;spam eggs&#39;
&#39;spam eggs&#39;
&gt;&gt;&gt; &#39;doesn\&#39;t&#39;
&quot;doesn&#39;t&quot;
&gt;&gt;&gt; &quot;doesn&#39;t&quot;
&quot;doesn&#39;t&quot;
&gt;&gt;&gt; &#39;&quot;Yes,&quot; he said.&#39;
&#39;&quot;Yes,&quot; he said.&#39;
&gt;&gt;&gt; &quot;\&quot;Yes,\&quot; he said.&quot;
&#39;&quot;Yes,&quot; he said.&#39;
&gt;&gt;&gt; &#39;&quot;Isn\&#39;t,&quot; she said.&#39;
&#39;&quot;Isn\&#39;t,&quot; she said.&#39;
</pre></div>
</div>
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&#8217;s enclosed in single quotes. The <a class="reference internal" href="../reference/simple_stmts.html#print"><tt class="xref std std-keyword docutils literal">print</tt></a> statement produces a more
readable output for such input strings.
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:
<div class="highlight-python"><div class="highlight"><pre>hello = &quot;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.&quot;

print hello
</pre></div>
</div>
Note that newlines still need to be embedded in the string using <tt class="docutils literal">\n</tt> &#8211; the
newline following the trailing backslash is discarded. This example would print
the following:
<div class="highlight-text"><div class="highlight"><pre>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.
</pre></div>
</div>
Or, strings can be surrounded in a pair of matching triple-quotes: <tt class="docutils literal">&quot;&quot;&quot;</tt> or
<tt class="docutils literal">'''</tt>. End of lines do not need to be escaped when using triple-quotes, but
they will be included in the string.
<div class="highlight-python"><div class="highlight"><pre>print &quot;&quot;&quot;
Usage: thingy [OPTIONS]
 -h Display this usage message
 -H hostname Hostname to connect to
&quot;&quot;&quot;
</pre></div>
</div>
produces the following output:
<div class="highlight-text"><div class="highlight"><pre>Usage: thingy [OPTIONS]
 -h Display this usage message
 -H hostname Hostname to connect to
</pre></div>
</div>
If we make the string literal a &#8220;raw&#8221; string, <tt class="docutils literal">\n</tt> 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:
<div class="highlight-python"><div class="highlight"><pre>hello = r&quot;This is a rather long string containing\n\
several lines of text much as you would do in C.&quot;

print hello
</pre></div>
</div>
would print:
<div class="highlight-text"><div class="highlight"><pre>This is a rather long string containing\n\
several lines of text much as you would do in C.
</pre></div>
</div>
Strings can be concatenated (glued together) with the <tt class="docutils literal">+</tt> operator, and
repeated with <tt class="docutils literal">*</tt>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; word = &#39;Help&#39; + &#39;A&#39;
&gt;&gt;&gt; word
&#39;HelpA&#39;
&gt;&gt;&gt; &#39;&lt;&#39; + word*5 + &#39;&gt;&#39;
&#39;&lt;HelpAHelpAHelpAHelpAHelpA&gt;&#39;
</pre></div>
</div>
Two string literals next to each other are automatically concatenated; the first
line above could also have been written <tt class="docutils literal">word = 'Help' 'A'</tt>; this only works
with two literals, not with arbitrary string expressions:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; &#39;str&#39; &#39;ing&#39; # &lt;- This is ok
&#39;string&#39;
&gt;&gt;&gt; &#39;str&#39;.strip() + &#39;ing&#39; # &lt;- This is ok
&#39;string&#39;
&gt;&gt;&gt; &#39;str&#39;.strip() &#39;ing&#39; # &lt;- This is invalid
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
 &#39;str&#39;.strip() &#39;ing&#39;
 ^
SyntaxError: invalid syntax
</pre></div>
</div>
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.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; word[4]
&#39;A&#39;
&gt;&gt;&gt; word[0:2]
&#39;He&#39;
&gt;&gt;&gt; word[2:4]
&#39;lp&#39;
</pre></div>
</div>
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.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; word[:2] # The first two characters
&#39;He&#39;
&gt;&gt;&gt; word[2:] # Everything except the first two characters
&#39;lpA&#39;
</pre></div>
</div>
Unlike a C string, Python strings cannot be changed. Assigning to an indexed
position in the string results in an error:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; word[0] = &#39;x&#39;
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
TypeError: object does not support item assignment
&gt;&gt;&gt; word[:1] = &#39;Splat&#39;
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
TypeError: object does not support slice assignment
</pre></div>
</div>
However, creating a new string with the combined content is easy and efficient:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; &#39;x&#39; + word[1:]
&#39;xelpA&#39;
&gt;&gt;&gt; &#39;Splat&#39; + word[4]
&#39;SplatA&#39;
</pre></div>
</div>
Here&#8217;s a useful invariant of slice operations: <tt class="docutils literal">s[:i] + s[i:]</tt> equals <tt class="docutils literal">s</tt>.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; word[:2] + word[2:]
&#39;HelpA&#39;
&gt;&gt;&gt; word[:3] + word[3:]
&#39;HelpA&#39;
</pre></div>
</div>
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.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; word[1:100]
&#39;elpA&#39;
&gt;&gt;&gt; word[10:]
&#39;&#39;
&gt;&gt;&gt; word[2:1]
&#39;&#39;
</pre></div>
</div>
Indices may be negative numbers, to start counting from the right. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; word[-1] # The last character
&#39;A&#39;
&gt;&gt;&gt; word[-2] # The last-but-one character
&#39;p&#39;
&gt;&gt;&gt; word[-2:] # The last two characters
&#39;pA&#39;
&gt;&gt;&gt; word[:-2] # Everything except the last two characters
&#39;Hel&#39;
</pre></div>
</div>
But note that -0 is really the same as 0, so it does not count from the right!
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; word[-0] # (since -0 equals 0)
&#39;H&#39;
</pre></div>
</div>
Out-of-range negative slice indices are truncated, but don&#8217;t try this for
single-element (non-slice) indices:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; word[-100:]
&#39;HelpA&#39;
&gt;&gt;&gt; word[-10] # error
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
IndexError: string index out of range
</pre></div>
</div>
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:
<div class="highlight-python"><pre> +---+---+---+---+---+
 | H | e | l | p | A |
 +---+---+---+---+---+
 0 1 2 3 4 5
-5 -4 -3 -2 -1</pre>
</div>
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 <tt class="docutils literal">word[1:3]</tt> is
2.
The built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><tt class="xref py py-func docutils literal">len()</tt></a> returns the length of a string:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; s = &#39;supercalifragilisticexpialidocious&#39;
&gt;&gt;&gt; len(s)
34
</pre></div>
</div>
<div class="admonition-see-also admonition seealso">
See also
<dl class="last docutils">
<dt><a class="reference internal" href="../library/stdtypes.html#typesseq">Sequence Types &#8212; str, unicode, list, tuple, bytearray, buffer, xrange</a></dt>
<dd>Strings, and the Unicode strings described in the next section, are
examples of sequence types, and support the common operations supported
by such types.</dd>
<dt><a class="reference internal" href="../library/stdtypes.html#string-methods">String Methods</a></dt>
<dd>Both strings and Unicode strings support a large number of methods for
basic transformations and searching.</dd>
<dt><a class="reference internal" href="../library/string.html#new-string-formatting">String Formatting</a></dt>
<dd>Information about string formatting with <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><tt class="xref py py-meth docutils literal">str.format()</tt></a> is described
here.</dd>
<dt><a class="reference internal" href="../library/stdtypes.html#string-formatting">String Formatting Operations</a></dt>
<dd>The old formatting operations invoked when strings and Unicode strings are
the left operand of the <tt class="docutils literal">%</tt> operator are described in more detail here.</dd>
</dl>
</div>
</div>
<div class="section" id="unicode-strings">
<h3>3.1.3. Unicode Strings<a class="headerlink" href="#unicode-strings" title="Permalink to this headline">¶</a></h3>
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 <a class="reference external" href="http://www.unicode.org/">http://www.unicode.org/</a>) 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
<tt class="docutils literal">i18n</tt> &#8212; <tt class="docutils literal">'i'</tt> + 18 characters + <tt class="docutils literal">'n'</tt>) 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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; u&#39;Hello World !&#39;
u&#39;Hello World !&#39;
</pre></div>
</div>
The small <tt class="docutils literal">'u'</tt> 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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; u&#39;Hello\u0020World !&#39;
u&#39;Hello World !&#39;
</pre></div>
</div>
The escape sequence <tt class="docutils literal">\u0020</tt> 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 &#8216;ur&#8217; to have Python use the
Raw-Unicode-Escape encoding. It will only apply the above <tt class="docutils literal">\uXXXX</tt>
conversion if there is an uneven number of backslashes in front of the small
&#8216;u&#8217;.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; ur&#39;Hello\u0020World !&#39;
u&#39;Hello World !&#39;
&gt;&gt;&gt; ur&#39;Hello\\u0020World !&#39;
u&#39;Hello\\\\u0020World !&#39;
</pre></div>
</div>
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 <a class="reference internal" href="../library/functions.html#unicode" title="unicode"><tt class="xref py py-func docutils literal">unicode()</tt></a> 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
<a class="reference internal" href="../library/functions.html#str" title="str"><tt class="xref py py-func docutils literal">str()</tt></a>, conversion takes place using this default encoding.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; u&quot;abc&quot;
u&#39;abc&#39;
&gt;&gt;&gt; str(u&quot;abc&quot;)
&#39;abc&#39;
&gt;&gt;&gt; u&quot;äöü&quot;
u&#39;\xe4\xf6\xfc&#39;
&gt;&gt;&gt; str(u&quot;äöü&quot;)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
UnicodeEncodeError: &#39;ascii&#39; codec can&#39;t encode characters in position 0-2: ordinal not in range(128)
</pre></div>
</div>
To convert a Unicode string into an 8-bit string using a specific encoding,
Unicode objects provide an <tt class="xref py py-func docutils literal">encode()</tt> method that takes one argument, the
name of the encoding. Lowercase names for encodings are preferred.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; u&quot;äöü&quot;.encode(&#39;utf-8&#39;)
&#39;\xc3\xa4\xc3\xb6\xc3\xbc&#39;
</pre></div>
</div>
If you have data in a specific encoding and want to produce a corresponding
Unicode string from it, you can use the <a class="reference internal" href="../library/functions.html#unicode" title="unicode"><tt class="xref py py-func docutils literal">unicode()</tt></a> function with the
encoding name as the second argument.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; unicode(&#39;\xc3\xa4\xc3\xb6\xc3\xbc&#39;, &#39;utf-8&#39;)
u&#39;\xe4\xf6\xfc&#39;
</pre></div>
</div>
</div>
<div class="section" id="lists">
<h3>3.1.4. Lists<a class="headerlink" href="#lists" title="Permalink to this headline">¶</a></h3>
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.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a = [&#39;spam&#39;, &#39;eggs&#39;, 100, 1234]
&gt;&gt;&gt; a
[&#39;spam&#39;, &#39;eggs&#39;, 100, 1234]
</pre></div>
</div>
Like string indices, list indices start at 0, and lists can be sliced,
concatenated and so on:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a[0]
&#39;spam&#39;
&gt;&gt;&gt; a[3]
1234
&gt;&gt;&gt; a[-2]
100
&gt;&gt;&gt; a[1:-1]
[&#39;eggs&#39;, 100]
&gt;&gt;&gt; a[:2] + [&#39;bacon&#39;, 2*2]
[&#39;spam&#39;, &#39;eggs&#39;, &#39;bacon&#39;, 4]
&gt;&gt;&gt; 3*a[:3] + [&#39;Boo!&#39;]
[&#39;spam&#39;, &#39;eggs&#39;, 100, &#39;spam&#39;, &#39;eggs&#39;, 100, &#39;spam&#39;, &#39;eggs&#39;, 100, &#39;Boo!&#39;]
</pre></div>
</div>
All slice operations return a new list containing the requested elements. This
means that the following slice returns a shallow copy of the list a:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a[:]
[&#39;spam&#39;, &#39;eggs&#39;, 100, 1234]
</pre></div>
</div>
Unlike strings, which are immutable, it is possible to change individual
elements of a list:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a
[&#39;spam&#39;, &#39;eggs&#39;, 100, 1234]
&gt;&gt;&gt; a[2] = a[2] + 23
&gt;&gt;&gt; a
[&#39;spam&#39;, &#39;eggs&#39;, 123, 1234]
</pre></div>
</div>
Assignment to slices is also possible, and this can even change the size of the
list or clear it entirely:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; # Replace some items:
... a[0:2] = [1, 12]
&gt;&gt;&gt; a
[1, 12, 123, 1234]
&gt;&gt;&gt; # Remove some:
... a[0:2] = []
&gt;&gt;&gt; a
[123, 1234]
&gt;&gt;&gt; # Insert some:
... a[1:1] = [&#39;bletch&#39;, &#39;xyzzy&#39;]
&gt;&gt;&gt; a
[123, &#39;bletch&#39;, &#39;xyzzy&#39;, 1234]
&gt;&gt;&gt; # Insert (a copy of) itself at the beginning
&gt;&gt;&gt; a[:0] = a
&gt;&gt;&gt; a
[123, &#39;bletch&#39;, &#39;xyzzy&#39;, 1234, 123, &#39;bletch&#39;, &#39;xyzzy&#39;, 1234]
&gt;&gt;&gt; # Clear the list: replace all items with an empty list
&gt;&gt;&gt; a[:] = []
&gt;&gt;&gt; a
[]
</pre></div>
</div>
The built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><tt class="xref py py-func docutils literal">len()</tt></a> also applies to lists:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a = [&#39;a&#39;, &#39;b&#39;, &#39;c&#39;, &#39;d&#39;]
&gt;&gt;&gt; len(a)
4
</pre></div>
</div>
It is possible to nest lists (create lists containing other lists), for
example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; q = [2, 3]
&gt;&gt;&gt; p = [1, q, 4]
&gt;&gt;&gt; len(p)
3
&gt;&gt;&gt; p[1]
[2, 3]
&gt;&gt;&gt; p[1][0]
2
&gt;&gt;&gt; p[1].append(&#39;xtra&#39;) # See section 5.1
&gt;&gt;&gt; p
[1, [2, 3, &#39;xtra&#39;], 4]
&gt;&gt;&gt; q
[2, 3, &#39;xtra&#39;]
</pre></div>
</div>
Note that in the last example, <tt class="docutils literal">p[1]</tt> and <tt class="docutils literal">q</tt> really refer to the same
object! We&#8217;ll come back to object semantics later.
</div>
</div>
<div class="section" id="first-steps-towards-programming">
<h2>3.2. First Steps Towards Programming<a class="headerlink" href="#first-steps-towards-programming" title="Permalink to this headline">¶</a></h2>
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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; # Fibonacci series:
... # the sum of two elements defines the next
... a, b = 0, 1
&gt;&gt;&gt; while b &lt; 10:
... print b
... a, b = b, a+b
...
1
1
2
3
5
8
</pre></div>
</div>
This example introduces several new features.
<ul>
<li>The first line contains a multiple assignment: the variables <tt class="docutils literal">a</tt> and <tt class="docutils literal">b</tt>
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.
</li>
<li>The <a class="reference internal" href="../reference/compound_stmts.html#while"><tt class="xref std std-keyword docutils literal">while</tt></a> loop executes as long as the condition (here: <tt class="docutils literal">b &lt; 10</tt>)
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: <tt class="docutils literal">&lt;</tt> (less than), <tt class="docutils literal">&gt;</tt> (greater than), <tt class="docutils literal">==</tt>
(equal to), <tt class="docutils literal">&lt;=</tt> (less than or equal to), <tt class="docutils literal">&gt;=</tt> (greater than or equal to)
and <tt class="docutils literal">!=</tt> (not equal to).
</li>
<li>The body of the loop is indented: indentation is Python&#8217;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.
</li>
<li>The <a class="reference internal" href="../reference/simple_stmts.html#print"><tt class="xref std std-keyword docutils literal">print</tt></a> 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:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; i = 256*256
&gt;&gt;&gt; print &#39;The value of i is&#39;, i
The value of i is 65536
</pre></div>
</div>
A trailing comma avoids the newline after the output:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a, b = 0, 1
&gt;&gt;&gt; while b &lt; 1000:
... print b,
... a, b = b, a+b
...
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
</pre></div>
</div>
Note that the interpreter inserts a newline before it prints the next prompt if
the last line was not completed.
</li>
</ul>
</div>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">3. An Informal Introduction to Python</a><ul>
<li><a class="reference internal" href="#using-python-as-a-calculator">3.1. Using Python as a Calculator</a><ul>
<li><a class="reference internal" href="#numbers">3.1.1. Numbers</a></li>
<li><a class="reference internal" href="#strings">3.1.2. Strings</a></li>
<li><a class="reference internal" href="#unicode-strings">3.1.3. Unicode Strings</a></li>
<li><a class="reference internal" href="#lists">3.1.4. Lists</a></li>
</ul>
</li>
<li><a class="reference internal" href="#first-steps-towards-programming">3.2. First Steps Towards Programming</a></li>
</ul>
</li>
</ul>

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