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copied!<p>General suggestions:</p> <ul> <li>Avoid unnecessary type checks, and rely on default exception behavior.</li> <li><a href="http://docs.python.org/library/stdtypes.html#dict.has_key" rel="noreferrer"><code>has_key()</code></a> has long been deprecated in favor of the <code>in</code> operator: use that instead.</li> <li><a href="http://docs.python.org/library/profile.html" rel="noreferrer">Profile</a> your program, before attempting any performance optimization. For a zero-effort profiling run of any given code, just run: <code>python -m cProfile -s cumulative foo.py</code></li> </ul> <p>Specific points:</p> <ul> <li><code>list</code> <a href="http://docs.python.org/tutorial/datastructures.html#using-lists-as-stacks" rel="noreferrer">makes a good stack</a> out of the box. In particular, it allows you to use <em>slice notation</em> (<a href="http://docs.python.org/tutorial/introduction.html#lists" rel="noreferrer">tutorial</a>) to replace the <code>pop</code>/<code>pop</code>/<code>append</code> dance with a single step.</li> <li><code>ARITHMETIC_OPERATORS</code> can refer to operator implementations directly, without the <code>getattr</code> indirection.</li> </ul> <p>Putting all this together:</p> <pre><code>ARITHMETIC_OPERATORS = { '+': operator.add, '-': operator.sub, '*': operator.mul, '/': operator.div, '%': operator.mod, '**': operator.pow, '//': operator.floordiv, } def postfix(expression, operators=ARITHMETIC_OPERATORS): stack = [] for val in expression.split(): if val in operators: f = operators[val] stack[-2:] = [f(*stack[-2:])] else: stack.append(int(val)) return stack.pop() </code></pre>

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