Why timeit doesn't work on my code snippet?

Question

I think these 3 are logically equivalent, returning the set `{1, 3, 4}`: ``` set(sum(((1, 3), (4,), (1,)), ())) set(sum([[1, 3], [4], [1]], [])) functools.reduce(operator.or_, ({1, 3}, {4}, {1}), set()) ``` But when I try to check the performance of each in ipython (v1.2.1 on python 3.4.0), the timeit magic fails. ``` In [1]: from operator import or_; from functools import reduce In [2]: timeit set(sum([[1, 3], [4], [1]], [])) 1000000 loops, best of 3: 604 ns per loop In [3]: timeit set(sum(((1, 3), (4,), (1,)), ())) 1000000 loops, best of 3: 330 ns per loop In [4]: timeit reduce(or_, ({1, 3}, {4}, {1}), set()) --------------------------------------------------------------------------- TypeError Traceback (most recent call last) in () ----> 1 get_ipython().magic('timeit reduce(or_, ({1, 3}, {4}, {1}), set())') /usr/lib/python3/dist-packages/IPython/core/interactiveshell.py in magic(self, arg_s) 2164 magic_name, _, magic_arg_s = arg_s.partition('…

Accepted Answer

This looks to me a bug in IPython.

First the workaround

escape the braces, so that the call looks like

```
timeit reduce(or_, ({{1, 3}}, {{4}}, {{1}}), set())
```

Now the Problem

If you see the call stack, before the call cascaded down to timeit.py, it passes through

```
/usr/lib/python3/dist-packages/IPython/core/interactiveshell.py in run_line_magic(self, magic_name, line)
   2085                 kwargs['local_ns'] = sys._getframe(stack_depth).f_locals
   2086             with self.builtin_trap:
-> 2087                 result = fn(*args,**kwargs)
   2088             return result
```

Now if you refer this source, you can see, that before the arguments get passed to the `timeit` function, its formatted to expand the Python variables in a string

```
        magic_arg_s = self.var_expand(line, stack_depth)
        # Put magic args in a list so we can call with f(*a) syntax
        args = [magic_arg_s]
```

`self.var_expand` calls `DollarFormatter()` as the for-matter function whose doc-string states something along the following lines

```
class DollarFormatter(FullEvalFormatter):
    """Formatter allowing Itpl style $foo replacement, for names and attribute
    access only. Standard {foo} replacement also works, and allows full
    evaluation of its arguments. 
```

So, that is the reason, a set is interpreted as a standard {foo} replacement and gets converted to either a tuple (if comma separated values) or a constant which makes the expression to

```
reduce(or_, ((1, 3), 4, 1), set())
```

which is off-course invalid.

Why timeit doesn't work on my code snippet?

Solution

Problem