April 26, 2014
2:58 p.m.
On 04/26/2014 09:54 AM, Chris Angelico wrote: > On Sat, Apr 26, 2014 at 11:43 PM, Philipp A.<flying-sheep@web.de> wrote: >> >sure it works if `eggs` has a `__iadd__` method. why shouldn’t it use the >> >outer local? > 1) Operator precedence gets in the way. (Easily fixed.) >>>> >>>lambda: eggs += lay_eggs() > SyntaxError: can't assign to lambda > > 2) Assignment is a statement, so you can't do it in a lambda. >>>> >>>lambda: (eggs += lay_eggs()) > SyntaxError: invalid syntax > > 3) Assignment makes it local, so you'll get UnboundLocalError if you > don't declare it nonlocal. >>>> >>>def f(): > eggs += lay_eggs() >>>> >>>f() > Traceback (most recent call last): > File "<pyshell#5>", line 1, in <module> > f() > File "<pyshell#4>", line 2, in f > eggs += lay_eggs() > UnboundLocalError: local variable 'eggs' referenced before assignment > > Hence my previous statement that you need to write the function out of > line, which breaks the switch-ness of the block, and you definitely > need to declare eggs nonlocal. Now, if eggs is a list, you can extend > it with a method (which can be called in an expression), but if your > switch block can only do expressions, it's pretty limited. Another difference is a typical switch hash map is usually built at compile time in other languages. In python, the example's dict would be built every time it executes. That can be an advantage at times, but it can also make it slower than an if-elif's Moving the dict (and function definitions) to a place where it's only initialised once, may mean the names it access may not be reachable, so in most cases any values it depends on need to be passed as well. Ron