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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-python3 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-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; 2 + 2
4
&gt;&gt;&gt; 50 - 5*6
20
&gt;&gt;&gt; (50 - 5*6) / 4
5.0
&gt;&gt;&gt; 8 / 5 # division always returns a floating point number
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.
Division (<code class="docutils literal notranslate">/</code>) always returns a float. To do <a class="reference internal" href="../glossary.html#term-floor-division">floor division</a> and
get an integer result (discarding any fractional result) you can use the <code class="docutils literal notranslate">//</code>
operator; to calculate the remainder you can use <code class="docutils literal notranslate">%</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; 17 / 3 # classic division returns a float
5.666666666666667
&gt;&gt;&gt;
&gt;&gt;&gt; 17 // 3 # floor division discards the fractional part
5
&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-python3 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-python3 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-python3 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-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; 4 * 3.75 - 1
14.0
</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-python3 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-python3 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="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate">print()</code></a> function produces a more
readable output, by omitting the enclosing quotes and by printing escaped
and special characters:
<div class="highlight-python3 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-python3 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-python3 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-python3 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-python3 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-python3 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-python3 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-python3 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-python3 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-python3 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 substring:
<div class="highlight-python3 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-python3 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-python3 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-python3 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-python3 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-python3 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-python3 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-python3 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-python3 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#textseq">Text Sequence Type — str</a></dt>
<dd>Strings 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>Strings support a large number of methods for
basic transformations and searching.</dd>
<dt><a class="reference internal" href="../reference/lexical_analysis.html#f-strings">Formatted string literals</a></dt>
<dd>String literals that have embedded expressions.</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#old-string-formatting">printf-style String Formatting</a></dt>
<dd>The old formatting operations invoked when 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="lists">
<h3>3.1.3. 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-python3 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-python3 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-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; squares[:]
[1, 4, 9, 16, 25]
</pre></div>
</div>
Lists also support operations like concatenation:
<div class="highlight-python3 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-python3 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-python3 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-python3 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-python3 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-python3 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-python3 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="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate">print()</code></a> function writes the value of the argument(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 arguments,
floating point quantities, and strings. Strings are printed without quotes,
and a space is inserted between items, so you can format things nicely, like
this:
<div class="highlight-python3 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>
The keyword argument end can be used to avoid the newline after the output,
or end the output with a different string:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; a, b = 0, 1
&gt;&gt;&gt; while b &lt; 1000:
... print(b, end=&#39;,&#39;)
... a, b = b, a+b
...
1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,
</pre></div>
</div>
</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>

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 <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="#lists">3.1.3. Lists</a></li>
</ul>
</li>
<li><a class="reference internal" href="#first-steps-towards-programming">3.2. First Steps Towards Programming</a></li>
</ul>
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</ul>

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