::  Posts  ::  RSS  ::  ◂◂RSS  ::  Contact

What is in hindsight a foolish way to do generator expressions

March 5th, 2009
python, programming  [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:

Incoming Gantz-Led Government to Invest in Israel’s Infrastructure

Israel’s incoming prime minister Benny Gantz unveiled an emergency government, to take power following an upcoming confidence vote in the Knesset. The last two MKs required to give Gantz a 61-59 majority, two members of Gantz’s own Blue and White Party wh…

via Pedestrian Observations April 1, 2020

Finding home in the time of coronavirus

Disclaimer: I’m going to say this once. Obviously, I am not happy about the coronavirus’s threat to public health or the economic toll it’s taking. I do not think the existence of this pandemic is good. Just so happens that social distancing and remote wo…

via Holly Elmore March 31, 2020

Massachusetts should shut down immediately

We’ve been running 30x fewer tests than other states, and have been extremely tardy in responding to the Biogen outbreak.

via benkuhn.net March 15, 2020

more     (via openring)

More Posts:


  ::  Posts  ::  RSS  ::  ◂◂RSS  ::  Contact