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| -rw-r--r-- | Doc/lib/libcollections.tex | 20 |
1 files changed, 12 insertions, 8 deletions
diff --git a/Doc/lib/libcollections.tex b/Doc/lib/libcollections.tex index 3e56a3e..a763e31 100644 --- a/Doc/lib/libcollections.tex +++ b/Doc/lib/libcollections.tex @@ -311,16 +311,20 @@ languages): When a letter is first encountered, it is missing from the mapping, so the \member{default_factory} function calls \function{int()} to supply a default count of zero. The increment operation then builds up the count for each -letter. This technique makes counting simpler and faster than an equivalent -technique using \method{dict.get()}: +letter. -\begin{verbatim} ->>> d = {} ->>> for k in s: - d[k] = d.get(k, 0) + 1 +The function \function{int()} which always returns zero is just a special +case of constant functions. A faster and more flexible way to create +constant functions is to use \function{itertools.repeat()} which can supply +any constant value (not just zero): ->>> d.items() -[('i', 4), ('p', 2), ('s', 4), ('m', 1)] +\begin{verbatim} +>>> def constant_factory(value): +... return itertools.repeat(value).next +>>> d = defaultdict(constant_factory('<missing>')) +>>> d.update(name='John', action='ran') +>>> '%(name)s %(action)s to %(object)s' % d +'John ran to <missing>' \end{verbatim} Setting the \member{default_factory} to \class{set} makes the |