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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 (<a class="reference internal" href="../glossary.html#term">&gt;&gt;&gt;</a> and <a class="reference internal" href="../glossary.html#term-1">…</a>): 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,
<code class="docutils literal notranslate">#</code>, 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-default notranslate"><div class="highlight"><pre># this is the first comment
spam = 1 # and this is the second comment
 # ... and now a third!
text = &quot;# This is not a comment because it&#39;s inside quotes.&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’s try some simple Python commands. Start the interpreter and wait for the
primary prompt, <code class="docutils literal notranslate">&gt;&gt;&gt;</code>. (It shouldn’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 <code class="docutils literal notranslate">+</code>, <code class="docutils literal notranslate">-</code>, <code class="docutils literal notranslate">*</code> and <code class="docutils literal notranslate">/</code> work just like in most other languages
(for example, Pascal or C); parentheses (<code class="docutils literal notranslate">()</code>) can be used for grouping.
For example:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; 2 + 2
4
&gt;&gt;&gt; 50 - 5*6
20
&gt;&gt;&gt; (50 - 5.0*6) / 4
5.0
&gt;&gt;&gt; 8 / 5.0
1.6
</pre></div>
</div>
The integer numbers (e.g. <code class="docutils literal notranslate">2</code>, <code class="docutils literal notranslate">4</code>, <code class="docutils literal notranslate">20</code>) have type <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a>,
the ones with a fractional part (e.g. <code class="docutils literal notranslate">5.0</code>, <code class="docutils literal notranslate">1.6</code>) have type
<a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate">float</code></a>. We will see more about numeric types later in the tutorial.
The return type of a division (<code class="docutils literal notranslate">/</code>) operation depends on its operands. If
both operands are of type <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a>, <a class="reference internal" href="../glossary.html#term-floor-division">floor division</a> is performed
and an <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a> is returned. If either operand is a <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate">float</code></a>,
classic division is performed and a <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate">float</code></a> is returned. The <code class="docutils literal notranslate">//</code>
operator is also provided for doing floor division no matter what the
operands are. The remainder can be calculated with the <code class="docutils literal notranslate">%</code> operator:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; 17 / 3 # int / int -&gt; int
5
&gt;&gt;&gt; 17 / 3.0 # int / float -&gt; float
5.666666666666667
&gt;&gt;&gt; 17 // 3.0 # explicit floor division discards the fractional part
5.0
&gt;&gt;&gt; 17 % 3 # the % operator returns the remainder of the division
2
&gt;&gt;&gt; 5 * 3 + 2 # result * divisor + remainder
17
</pre></div>
</div>
With Python, it is possible to use the <code class="docutils literal notranslate">**</code> operator to calculate powers <a class="footnote-reference" href="#id3" id="id1">[1]</a>:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; 5 ** 2 # 5 squared
25
&gt;&gt;&gt; 2 ** 7 # 2 to the power of 7
128
</pre></div>
</div>
The equal sign (<code class="docutils literal notranslate">=</code>) is used to assign a value to a variable. Afterwards, no
result is displayed before the next interactive prompt:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; width = 20
&gt;&gt;&gt; height = 5 * 9
&gt;&gt;&gt; width * height
900
</pre></div>
</div>
If a variable is not “defined” (assigned a value), trying to use it will
give you an error:
<div class="highlight-default notranslate"><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-default notranslate"><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>
In interactive mode, the last printed expression is assigned to the variable
<code class="docutils literal notranslate">_</code>. This means that when you are using Python as a desk calculator, it is
somewhat easier to continue calculations, for example:
<div class="highlight-default notranslate"><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’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 <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a> and <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate">float</code></a>, Python supports other types of
numbers, such as <a class="reference internal" href="../library/decimal.html#decimal.Decimal" title="decimal.Decimal"><code class="xref py py-class docutils literal notranslate">Decimal</code></a> and <a class="reference internal" href="../library/fractions.html#fractions.Fraction" title="fractions.Fraction"><code class="xref py py-class docutils literal notranslate">Fraction</code></a>.
Python also has built-in support for <a class="reference internal" href="../library/stdtypes.html#typesnumeric">complex numbers</a>,
and uses the <code class="docutils literal notranslate">j</code> or <code class="docutils literal notranslate">J</code> suffix to indicate the imaginary part
(e.g. <code class="docutils literal notranslate">3+5j</code>).
</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 (<code class="docutils literal notranslate">'...'</code>) or
double quotes (<code class="docutils literal notranslate">&quot;...&quot;</code>) with the same result <a class="footnote-reference" href="#id4" id="id2">[2]</a>. <code class="docutils literal notranslate">\</code> can be used
to escape quotes:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; &#39;spam eggs&#39; # single quotes
&#39;spam eggs&#39;
&gt;&gt;&gt; &#39;doesn\&#39;t&#39; # use \&#39; to escape the single quote...
&quot;doesn&#39;t&quot;
&gt;&gt;&gt; &quot;doesn&#39;t&quot; # ...or use double quotes instead
&quot;doesn&#39;t&quot;
&gt;&gt;&gt; &#39;&quot;Yes,&quot; they said.&#39;
&#39;&quot;Yes,&quot; they said.&#39;
&gt;&gt;&gt; &quot;\&quot;Yes,\&quot; they said.&quot;
&#39;&quot;Yes,&quot; they said.&#39;
&gt;&gt;&gt; &#39;&quot;Isn\&#39;t,&quot; they said.&#39;
&#39;&quot;Isn\&#39;t,&quot; they said.&#39;
</pre></div>
</div>
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 <a class="reference internal" href="../reference/simple_stmts.html#print"><code class="xref std std-keyword docutils literal notranslate">print</code></a> statement produces a more
readable output, by omitting the enclosing quotes and by printing escaped
and special characters:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; &#39;&quot;Isn\&#39;t,&quot; they said.&#39;
&#39;&quot;Isn\&#39;t,&quot; they said.&#39;
&gt;&gt;&gt; print &#39;&quot;Isn\&#39;t,&quot; they said.&#39;
&quot;Isn&#39;t,&quot; they said.
&gt;&gt;&gt; s = &#39;First line.\nSecond line.&#39; # \n means newline
&gt;&gt;&gt; s # without print, \n is included in the output
&#39;First line.\nSecond line.&#39;
&gt;&gt;&gt; print s # with print, \n produces a new line
First line.
Second line.
</pre></div>
</div>
If you don’t want characters prefaced by <code class="docutils literal notranslate">\</code> to be interpreted as
special characters, you can use raw strings by adding an <code class="docutils literal notranslate">r</code> before
the first quote:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print &#39;C:\some\name&#39; # here \n means newline!
C:\some
ame
&gt;&gt;&gt; print r&#39;C:\some\name&#39; # note the r before the quote
C:\some\name
</pre></div>
</div>
String literals can span multiple lines. One way is using triple-quotes:
<code class="docutils literal notranslate">&quot;&quot;&quot;...&quot;&quot;&quot;</code> or <code class="docutils literal notranslate">'''...'''</code>. End of lines are automatically
included in the string, but it’s possible to prevent this by adding a <code class="docutils literal notranslate">\</code> at
the end of the line. The following example:
<div class="highlight-default notranslate"><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 (note that the initial newline is not included):
<div class="highlight-text notranslate"><div class="highlight"><pre>Usage: thingy [OPTIONS]
 -h Display this usage message
 -H hostname Hostname to connect to
</pre></div>
</div>
Strings can be concatenated (glued together) with the <code class="docutils literal notranslate">+</code> operator, and
repeated with <code class="docutils literal notranslate">*</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; # 3 times &#39;un&#39;, followed by &#39;ium&#39;
&gt;&gt;&gt; 3 * &#39;un&#39; + &#39;ium&#39;
&#39;unununium&#39;
</pre></div>
</div>
Two or more string literals (i.e. the ones enclosed between quotes) next
to each other are automatically concatenated.
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; &#39;Py&#39; &#39;thon&#39;
&#39;Python&#39;
</pre></div>
</div>
This feature is particularly useful when you want to break long strings:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; text = (&#39;Put several strings within parentheses &#39;
... &#39;to have them joined together.&#39;)
&gt;&gt;&gt; text
&#39;Put several strings within parentheses to have them joined together.&#39;
</pre></div>
</div>
This only works with two literals though, not with variables or expressions:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; prefix = &#39;Py&#39;
&gt;&gt;&gt; prefix &#39;thon&#39; # can&#39;t concatenate a variable and a string literal
 ...
SyntaxError: invalid syntax
&gt;&gt;&gt; (&#39;un&#39; * 3) &#39;ium&#39;
 ...
SyntaxError: invalid syntax
</pre></div>
</div>
If you want to concatenate variables or a variable and a literal, use <code class="docutils literal notranslate">+</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; prefix + &#39;thon&#39;
&#39;Python&#39;
</pre></div>
</div>
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:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; word = &#39;Python&#39;
&gt;&gt;&gt; word[0] # character in position 0
&#39;P&#39;
&gt;&gt;&gt; word[5] # character in position 5
&#39;n&#39;
</pre></div>
</div>
Indices may also be negative numbers, to start counting from the right:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; word[-1] # last character
&#39;n&#39;
&gt;&gt;&gt; word[-2] # second-last character
&#39;o&#39;
&gt;&gt;&gt; word[-6]
&#39;P&#39;
</pre></div>
</div>
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:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; word[0:2] # characters from position 0 (included) to 2 (excluded)
&#39;Py&#39;
&gt;&gt;&gt; word[2:5] # characters from position 2 (included) to 5 (excluded)
&#39;tho&#39;
</pre></div>
</div>
Note how the start is always included, and the end always excluded. This
makes sure that <code class="docutils literal notranslate">s[:i] + s[i:]</code> is always equal to <code class="docutils literal notranslate">s</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; word[:2] + word[2:]
&#39;Python&#39;
&gt;&gt;&gt; word[:4] + word[4:]
&#39;Python&#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-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; word[:2] # character from the beginning to position 2 (excluded)
&#39;Py&#39;
&gt;&gt;&gt; word[4:] # characters from position 4 (included) to the end
&#39;on&#39;
&gt;&gt;&gt; word[-2:] # characters from the second-last (included) to the end
&#39;on&#39;
</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-default notranslate"><div class="highlight"><pre> +---+---+---+---+---+---+
 | P | y | t | h | o | n |
 +---+---+---+---+---+---+
 0 1 2 3 4 5 6
-6 -5 -4 -3 -2 -1
</pre></div>
</div>
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 <code class="docutils literal notranslate">word[1:3]</code> is
2.
Attempting to use an index that is too large will result in an error:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; word[42] # the word only has 6 characters
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
IndexError: string index out of range
</pre></div>
</div>
However, out of range slice indexes are handled gracefully when used for
slicing:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; word[4:42]
&#39;on&#39;
&gt;&gt;&gt; word[42:]
&#39;&#39;
</pre></div>
</div>
Python strings cannot be changed — they are <a class="reference internal" href="../glossary.html#term-immutable">immutable</a>.
Therefore, assigning to an indexed position in the string results in an error:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; word[0] = &#39;J&#39;
 ...
TypeError: &#39;str&#39; object does not support item assignment
&gt;&gt;&gt; word[2:] = &#39;py&#39;
 ...
TypeError: &#39;str&#39; object does not support item assignment
</pre></div>
</div>
If you need a different string, you should create a new one:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; &#39;J&#39; + word[1:]
&#39;Jython&#39;
&gt;&gt;&gt; word[:2] + &#39;py&#39;
&#39;Pypy&#39;
</pre></div>
</div>
The built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate">len()</code></a> returns the length of a string:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; s = &#39;supercalifragilisticexpialidocious&#39;
&gt;&gt;&gt; len(s)
34
</pre></div>
</div>
<div class="admonition seealso">
See also
<dl class="last docutils">
<dt><a class="reference internal" href="../library/stdtypes.html#typesseq">Sequence Types — 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#formatstrings">Format String Syntax</a></dt>
<dd>Information about string formatting with <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate">str.format()</code></a>.</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 <code class="docutils literal notranslate">%</code> 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
<code class="docutils literal notranslate">i18n</code> — <code class="docutils literal notranslate">'i'</code> + 18 characters + <code class="docutils literal notranslate">'n'</code>) 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-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; u&#39;Hello World !&#39;
u&#39;Hello World !&#39;
</pre></div>
</div>
The small <code class="docutils literal notranslate">'u'</code> 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-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; u&#39;Hello\u0020World !&#39;
u&#39;Hello World !&#39;
</pre></div>
</div>
The escape sequence <code class="docutils literal notranslate">\u0020</code> 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 <code class="docutils literal notranslate">\uXXXX</code>
conversion if there is an uneven number of backslashes in front of the small
‘u’.
<div class="highlight-default notranslate"><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"><code class="xref py py-func docutils literal notranslate">unicode()</code></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"><code class="xref py py-func docutils literal notranslate">str()</code></a>, conversion takes place using this default encoding.
<div class="highlight-default notranslate"><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 <code class="xref py py-func docutils literal notranslate">encode()</code> method that takes one argument, the
name of the encoding. Lowercase names for encodings are preferred.
<div class="highlight-default notranslate"><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"><code class="xref py py-func docutils literal notranslate">unicode()</code></a> function with the
encoding name as the second argument.
<div class="highlight-default notranslate"><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. Lists might contain
items of different types, but usually the items all have the same type.
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; squares = [1, 4, 9, 16, 25]
&gt;&gt;&gt; squares
[1, 4, 9, 16, 25]
</pre></div>
</div>
Like strings (and all other built-in <a class="reference internal" href="../glossary.html#term-sequence">sequence</a> type), lists can be
indexed and sliced:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; squares[0] # indexing returns the item
1
&gt;&gt;&gt; squares[-1]
25
&gt;&gt;&gt; squares[-3:] # slicing returns a new list
[9, 16, 25]
</pre></div>
</div>
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:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; squares[:]
[1, 4, 9, 16, 25]
</pre></div>
</div>
Lists also supports operations like concatenation:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; squares + [36, 49, 64, 81, 100]
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
</pre></div>
</div>
Unlike strings, which are <a class="reference internal" href="../glossary.html#term-immutable">immutable</a>, lists are a <a class="reference internal" href="../glossary.html#term-mutable">mutable</a>
type, i.e. it is possible to change their content:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; cubes = [1, 8, 27, 65, 125] # something&#39;s wrong here
&gt;&gt;&gt; 4 ** 3 # the cube of 4 is 64, not 65!
64
&gt;&gt;&gt; cubes[3] = 64 # replace the wrong value
&gt;&gt;&gt; cubes
[1, 8, 27, 64, 125]
</pre></div>
</div>
You can also add new items at the end of the list, by using
the <code class="xref py py-meth docutils literal notranslate">append()</code> method (we will see more about methods later):
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; cubes.append(216) # add the cube of 6
&gt;&gt;&gt; cubes.append(7 ** 3) # and the cube of 7
&gt;&gt;&gt; cubes
[1, 8, 27, 64, 125, 216, 343]
</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-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; letters = [&#39;a&#39;, &#39;b&#39;, &#39;c&#39;, &#39;d&#39;, &#39;e&#39;, &#39;f&#39;, &#39;g&#39;]
&gt;&gt;&gt; letters
[&#39;a&#39;, &#39;b&#39;, &#39;c&#39;, &#39;d&#39;, &#39;e&#39;, &#39;f&#39;, &#39;g&#39;]
&gt;&gt;&gt; # replace some values
&gt;&gt;&gt; letters[2:5] = [&#39;C&#39;, &#39;D&#39;, &#39;E&#39;]
&gt;&gt;&gt; letters
[&#39;a&#39;, &#39;b&#39;, &#39;C&#39;, &#39;D&#39;, &#39;E&#39;, &#39;f&#39;, &#39;g&#39;]
&gt;&gt;&gt; # now remove them
&gt;&gt;&gt; letters[2:5] = []
&gt;&gt;&gt; letters
[&#39;a&#39;, &#39;b&#39;, &#39;f&#39;, &#39;g&#39;]
&gt;&gt;&gt; # clear the list by replacing all the elements with an empty list
&gt;&gt;&gt; letters[:] = []
&gt;&gt;&gt; letters
[]
</pre></div>
</div>
The built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate">len()</code></a> also applies to lists:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; letters = [&#39;a&#39;, &#39;b&#39;, &#39;c&#39;, &#39;d&#39;]
&gt;&gt;&gt; len(letters)
4
</pre></div>
</div>
It is possible to nest lists (create lists containing other lists), for
example:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; a = [&#39;a&#39;, &#39;b&#39;, &#39;c&#39;]
&gt;&gt;&gt; n = [1, 2, 3]
&gt;&gt;&gt; x = [a, n]
&gt;&gt;&gt; x
[[&#39;a&#39;, &#39;b&#39;, &#39;c&#39;], [1, 2, 3]]
&gt;&gt;&gt; x[0]
[&#39;a&#39;, &#39;b&#39;, &#39;c&#39;]
&gt;&gt;&gt; x[0][1]
&#39;b&#39;
</pre></div>
</div>
</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-default notranslate"><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 <code class="docutils literal notranslate">a</code> and <code class="docutils literal notranslate">b</code>
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"><code class="xref std std-keyword docutils literal notranslate">while</code></a> loop executes as long as the condition (here: <code class="docutils literal notranslate">b &lt; 10</code>)
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: <code class="docutils literal notranslate">&lt;</code> (less than), <code class="docutils literal notranslate">&gt;</code> (greater than), <code class="docutils literal notranslate">==</code>
(equal to), <code class="docutils literal notranslate">&lt;=</code> (less than or equal to), <code class="docutils literal notranslate">&gt;=</code> (greater than or equal to)
and <code class="docutils literal notranslate">!=</code> (not equal to).
</li>
<li>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.
</li>
<li>The <a class="reference internal" href="../reference/simple_stmts.html#print"><code class="xref std std-keyword docutils literal notranslate">print</code></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-default notranslate"><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-default notranslate"><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>
Footnotes
<table class="docutils footnote" frame="void" id="id3" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>Since <code class="docutils literal notranslate">**</code> has higher precedence than <code class="docutils literal notranslate">-</code>, <code class="docutils literal notranslate">-3**2</code> will be
interpreted as <code class="docutils literal notranslate">-(3**2)</code> and thus result in <code class="docutils literal notranslate">-9</code>. To avoid this
and get <code class="docutils literal notranslate">9</code>, you can use <code class="docutils literal notranslate">(-3)**2</code>.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id4" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id2">[2]</a></td><td>Unlike other languages, special characters such as <code class="docutils literal notranslate">\n</code> have the
same meaning with both single (<code class="docutils literal notranslate">'...'</code>) and double (<code class="docutils literal notranslate">&quot;...&quot;</code>) quotes.
The only difference between the two is that within single quotes you don’t
need to escape <code class="docutils literal notranslate">&quot;</code> (but you have to escape <code class="docutils literal notranslate">\'</code>) and vice versa.</td></tr>
</tbody>
</table>
</div>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
 <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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