4 ms·

state = [st for s in state for st in states[(s,letter)]] can be read as new_state = [] for s in state: for st in states[(s, letter)]:

state = [st for s in state for st in states[(s,letter)]] can be read as new_state = [] for s in state: for st in states[(s, letter)]: new_state.append(st) state = new_state

lmm 15y ago
I'm a big python user and I didn't realize this. It seems like the nesting order for list comprehensions is backwards? I would naturally read the line as state = [st for s in [state for st in states[(s,letter)]]] because that's the way the natural language brackets, but this line is obviously broken. I would expect the syntax for the for loop you write to look like "state = [st for st in states(s, letter) for s in state]", which I think reads very clearly and understandably. So can anyone explain why the nesting works the way it does?
- divtxt 15y ago
  I too expected the syntax list comprehensions to chained. Turns out that nesting is a feature: http://www.python.org/dev/peps/pep-0202/ http://www.python.org/dev/peps/pep-0202/ - The form [... for x... for y...] nests, with the last index varying fastest, just like nested for loops.
- ajanuary 15y ago
  My background is with LINQ, which means I find it more natural to group it as: st {for s in state} {for st in states[(s,letter)]} But I agree, reading it as English your grouping makes more sense.
- evmar 15y ago
  This got me too -- I thought the reason the code was "the strangest" was because the double-loop was nested the wrong way.
acqq 15y ago
And here is an example how it actually runs. I did some renames to make more obvious what's where: something = [ x for loop ] Means something = [] for loop: something.append[ x ] so I'll use x for the current element of the resulting list (I'll rename st to x). Second, the 'letter' is not relevant for analysis, I'll assume a constant. Third, there are variables 'state' and 'states' so naming again can confusingly suggest that there is some deep connection between them. I'll rename 'states' to 'moves'. So instead of: state = [st for s in state for st in states[(s,letter)]] I'll start from: state = [x for s in state for x in moves[s]] Now to show exactly what's going on, I'll rewrite it to print partial results: state=[0,1,2] moves=[[11,12],[21,22],[31,32]] t = [] for s in state: a = moves[ s ] print "s, a:",s, a for x in a: print "x:", x t.append( x ) print t state = t The output is: s, a: 0 [11, 12] x: 11 x: 12 s, a: 1 [21, 22] x: 21 x: 22 s, a: 2 [31, 32] x: 31 x: 32 [11, 12, 21, 22, 31, 32]

state = [st for s in state for st in states[(s,letter)]] can be read as new_state = [] for s in state: for st in states[(s, letter)]: new_state.append(st) state = new_state

lmm 15y ago

I'm a big python user and I didn't realize this. It seems like the nesting order for list comprehensions is backwards? I would naturally read the line as state = [st for s in [state for st in states[(s,letter)]]] because that's the way the natural language brackets, but this line is obviously broken. I would expect the syntax for the for loop you write to look like "state = [st for st in states(s, letter) for s in state]", which I think reads very clearly and understandably. So can anyone explain why the nesting works the way it does?

divtxt 15y ago
I too expected the syntax list comprehensions to chained. Turns out that nesting is a feature: http://www.python.org/dev/peps/pep-0202/ http://www.python.org/dev/peps/pep-0202/ - The form [... for x... for y...] nests, with the last index varying fastest, just like nested for loops.
ajanuary 15y ago
My background is with LINQ, which means I find it more natural to group it as: st {for s in state} {for st in states[(s,letter)]} But I agree, reading it as English your grouping makes more sense.
evmar 15y ago
This got me too -- I thought the reason the code was "the strangest" was because the double-loop was nested the wrong way.

acqq 15y ago

And here is an example how it actually runs. I did some renames to make more obvious what's where: something = [ x for loop ] Means something = [] for loop: something.append[ x ] so I'll use x for the current element of the resulting list (I'll rename st to x). Second, the 'letter' is not relevant for analysis, I'll assume a constant. Third, there are variables 'state' and 'states' so naming again can confusingly suggest that there is some deep connection between them. I'll rename 'states' to 'moves'. So instead of: state = [st for s in state for st in states[(s,letter)]] I'll start from: state = [x for s in state for x in moves[s]] Now to show exactly what's going on, I'll rewrite it to print partial results: state=[0,1,2] moves=[[11,12],[21,22],[31,32]] t = [] for s in state: a = moves[ s ] print "s, a:",s, a for x in a: print "x:", x t.append( x ) print t state = t The output is: s, a: 0 [11, 12] x: 11 x: 12 s, a: 1 [21, 22] x: 21 x: 22 s, a: 2 [31, 32] x: 31 x: 32 [11, 12, 21, 22, 31, 32]