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<H2>
<BR>
2.1.6 Mapping Types 
</H2>


<P>
A <i class="dfn">mapping</i> object maps values of one type (the key type) to
arbitrary objects.  Mappings are mutable objects.  There is currently
only one standard mapping type, the <i class="dfn">dictionary</i>.  A dictionary's keys are
almost arbitrary values.  The only types of values not acceptable as
keys are values containing lists or dictionaries or other mutable
types that are compared by value rather than by object identity.
Numeric types used for keys obey the normal rules for numeric
comparison: if two numbers compare equal (e.g. <code>1</code> and
<code>1.0</code>) then they can be used interchangeably to index the same
dictionary entry.

<P>
Dictionaries are created by placing a comma-separated list of
<code><var>key</var>: <var>value</var></code> pairs within braces, for example:
<code>{'jack': 4098, 'sjoerd': 4127}</code> or
<code>{4098: 'jack', 4127: 'sjoerd'}</code>.

<P>
The following operations are defined on mappings (where <var>a</var> and
<var>b</var> are mappings, <var>k</var> is a key, and <var>v</var> and <var>x</var> are
arbitrary objects):


  
<P>
<table border align="center" style="border-collapse: collapse">
  <thead>
    <tr>
      <th align="center"><b>Operation</b>&nbsp;</th>
      <th align="left"><b>Result</b>&nbsp;</th>
      <th align="center"><b>Notes</b>&nbsp;</th>
    </thead>
  <tbody valign='baseline'>
    <tr><td align="center" valign="baseline"><code>len(<var>a</var>)</code></td>
        <td align="left">the number of items in <var>a</var></td>
        <td align="center">&nbsp;</td>
    <tr><td align="center" valign="baseline"><code><var>a</var>[<var>k</var>]</code></td>
        <td align="left">the item of <var>a</var> with key <var>k</var></td>
        <td align="center">(1)</td>
    <tr><td align="center" valign="baseline"><code><var>a</var>[<var>k</var>] = <var>v</var></code></td>
        <td align="left">set <code><var>a</var>[<var>k</var>]</code> to <var>v</var></td>
        <td align="center">&nbsp;</td>
    <tr><td align="center" valign="baseline"><code>del <var>a</var>[<var>k</var>]</code></td>
        <td align="left">remove <code><var>a</var>[<var>k</var>]</code> from <var>a</var></td>
        <td align="center">(1)</td>
    <tr><td align="center" valign="baseline"><code><var>a</var>.clear()</code></td>
        <td align="left">remove all items from <code>a</code></td>
        <td align="center">&nbsp;</td>
    <tr><td align="center" valign="baseline"><code><var>a</var>.copy()</code></td>
        <td align="left">a (shallow) copy of <code>a</code></td>
        <td align="center">&nbsp;</td>
    <tr><td align="center" valign="baseline"><code><var>a</var>.has_key(<var>k</var>)</code></td>
        <td align="left"><code>1</code> if <var>a</var> has a key <var>k</var>, else <code>0</code></td>
        <td align="center">&nbsp;</td>
    <tr><td align="center" valign="baseline"><code><var>a</var>.items()</code></td>
        <td align="left">a copy of <var>a</var>'s list of (<var>key</var>, <var>value</var>) pairs</td>
        <td align="center">(2)</td>
    <tr><td align="center" valign="baseline"><code><var>a</var>.keys()</code></td>
        <td align="left">a copy of <var>a</var>'s list of keys</td>
        <td align="center">(2)</td>
    <tr><td align="center" valign="baseline"><code><var>a</var>.update(<var>b</var>)</code></td>
        <td align="left"><code>for k in <var>b</var>.keys(): <var>a</var>[k] = <var>b</var>[k]</code></td>
        <td align="center">(3)</td>
    <tr><td align="center" valign="baseline"><code><var>a</var>.values()</code></td>
        <td align="left">a copy of <var>a</var>'s list of values</td>
        <td align="center">(2)</td>
    <tr><td align="center" valign="baseline"><code><var>a</var>.get(<var>k</var><big>[</big>, <var>x</var><big>]</big>)</code></td>
        <td align="left"><code><var>a</var>[<var>k</var>]</code> if <code><var>a</var>.has_key(<var>k</var>)</code>,
           else <var>x</var></td>
        <td align="center">(4)</td>
    <tr><td align="center" valign="baseline"><code><var>a</var>.setdefault(<var>k</var><big>[</big>, <var>x</var><big>]</big>)</code></td>
        <td align="left"><code><var>a</var>[<var>k</var>]</code> if <code><var>a</var>.has_key(<var>k</var>)</code>,
           else <var>x</var> (also setting it)</td>
        <td align="center">(5)</td></tbody>
</table>

<P>
Notes:
<DL>
<DT><STRONG>(1)</STRONG></DT>
<DD>Raises a <tt class="exception">KeyError</tt> exception if <var>k</var> is not
in the map.

<P>
</DD>
<DT><STRONG>(2)</STRONG></DT>
<DD>Keys and values are listed in random order.  If
<tt class="method">keys()</tt> and <tt class="method">values()</tt> are called with no intervening
modifications to the dictionary, the two lists will directly
correspond.  This allows the creation of <code>(<var>value</var>,
<var>key</var>)</code> pairs using <tt class="function">map()</tt>: "<tt class="samp">pairs = map(None,
<var>a</var>.values(), <var>a</var>.keys())</tt>".

<P>
</DD>
<DT><STRONG>(3)</STRONG></DT>
<DD><var>b</var> must be of the same type as <var>a</var>.

<P>
</DD>
<DT><STRONG>(4)</STRONG></DT>
<DD>Never raises an exception if <var>k</var> is not in the map,
instead it returns <var>x</var>.  <var>x</var> is optional; when <var>x</var> is not
provided and <var>k</var> is not in the map, <code>None</code> is returned.

<P>
</DD>
<DT><STRONG>(5)</STRONG></DT>
<DD><tt class="function">setdefault()</tt> is like <tt class="function">get()</tt>, except
that if <var>k</var> is missing, <var>x</var> is both returned and inserted into
the dictionary as the value of <var>k</var>.
</DD>
</DL>

<P>

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