• Posts
  • RSS
  • ◂◂RSS
  • Contact

  • What is in hindsight a foolish way to do generator expressions

    March 5th, 2009
    programming, python  [html]
    In python I often want to do things like this:
           if x.startswith("foo") for all x in a_list:
              ...
    
           if is_verb(leaf) for some leaf in a_tree:
              ...
        
    I usually end up coding these a bunch of different ways, the shortest of which are:
            if False not in [x.startswith("foo") for x in a_list]:
               ...
    
            if True in [is_verb(leaf) for leaf in a_tree]:
               ...
        
    But these don't short circut and are ugly. Sometimes I'll write little funtions but I don't like that either. Currently the syntax if [expr] for ... doesn't mean anything, so this should be possible. I wonder if it would be worth writing a PEP.

    Update 3/6/2009: George Dahl writes that python already does this. I should be writing those examples as:

           if all(x.startswith("foo") for x in a_list):
              ...
    
           if any(is_verb(leaf) for leaf in a_tree):
              ...
        
    These examples use generator expressions instead of list comprehensions. And builtins any and all. Especially as one can use any function, not just any or all, this is a much nicer syntax than what I wanted. It doesn't read as fluidly as english, but then again it doesn't deal with these two specific case as special cases of the if statement. So I'm quite happy.

    The only other real downside is also I think the reason I didn't end up with this when fiddling around at the python prompt: you need somewhat non-intuitive parens. I'm pretty sure when I was trying to see if there was a way to do what I wanted I did:

        >>> x for x in range(7)
          File "<stdin>", line 1
            x for x in range(7)
                ^
          SyntaxError: invalid syntax
        
    And then moved on. As a human I don't see an obvious reason why I can't enter the obvious thing. Is there something else a statement beginning with "x for " could mean?

    Comment via: facebook

    Recent posts on blogs I like:

    Experiences in raising children in shared housing

    Sometimes I see posts about people’s hope to raise children in a group housing situation, and it often seems overly optimistic to me. In particular they seem to expect that there will be more shared childcare than I think should be expected. Today I talke…

    via The whole sky October 18, 2021

    What to learn

    It's common to see people advocate for learning skills that they have or using processes that they use. For example, Steve Yegge has a set of blog posts where he recommends reading compiler books and learning about compilers. His reasoning is basicall…

    via Posts on Dan Luu October 18, 2021

    EDT with updating double counts

    I recently got confused thinking about the following case: Calculator bet: I am offered the opportunity to bet on a mathematical statement X to which I initially assign 50% probability (perhaps X = 139926 is a quadratic residue modulo 314159). I have acce…

    via The sideways view October 12, 2021

    more     (via openring)


  • Posts
  • RSS
  • ◂◂RSS
  • Contact