Re: Best way to check that you are at the beginning (the end) of an iterable?
Cameron Simpson wrote: About the only time I do this is my personal the() convenience function: def the(list, context=None): ''' Returns the first element of an iterable, but requires there to be exactly one. ''' icontext=expected exactly one value if context is not None: icontext=icontext+ for +context first=True for elem in list: if first: it=elem first=False else: raise IndexError, %s: got more than one element (%s, %s, ...) \ % (icontext, it, elem) if first: raise IndexError, %s: got no elements % icontext return it Which I use as a definite article in places where an iterable should yield exactly one result (eg SQL SELECTs that ought to get exactly one hit). I can see I wrote that a long time ago - it could do with some style fixes. And a code scan shows it sees little use:-) A lightweight alternative to that is unpacking: [x] = Traceback (most recent call last): File stdin, line 1, in module ValueError: need more than 0 values to unpack [x] = a [x] = ab Traceback (most recent call last): File stdin, line 1, in module ValueError: too many values to unpack -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
On Fri, Sep 9, 2011 at 9:04 PM, Peter Otten __pete...@web.de wrote: [x] = Traceback (most recent call last): File stdin, line 1, in module ValueError: need more than 0 values to unpack [x] = a [x] = ab Traceback (most recent call last): File stdin, line 1, in module ValueError: too many values to unpack Hey look, it's a new operator - the assign-sole-result-of-iterable operator! x ,= a :) ChrisA -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
In article mailman.854.1315441399.27778.python-l...@python.org Cameron Simpson c...@zip.com.au wrote: Facilities like feof() in C and eof in Pascal already lead to lots of code that runs happily with flat files and behaves badly in interactive or piped input. It is _so_ easy to adopt a style like: while not eof(filehandle): line = filehandle.nextline() ... Minor but important point here: eof() in Pascal is predictive (uses a crystal ball to peer into the future to see whether EOF is is about to occur -- which really means, reads ahead, causing that interactivity problem you mentioned), but feof() in C is post-dictive. The feof(stream) function returns a false value if the stream has not yet encountered an EOF, but your very next attempt to read from it may (or may not) immediately encounter that EOF. Thus, feof() in C is sort of (but not really) useless. (The actual use cases are to distinguish between EOF and error after a failed read from a stream -- since C lacks exceptions, getc() just returns EOF to indicate failed to get a character due to end of file or error -- or in some more obscure cases, such as the nonstandard getw(), to distinguish between a valid -1 value and having encountered an EOF. The companion ferror() function tells you whether an earlier EOF value was due to an error.) -- In-Real-Life: Chris Torek, Wind River Systems Intel require I note that my opinions are not those of WRS or Intel Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603 email: gmail (figure it out) http://web.torek.net/torek/index.html -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
On 08Sep2011 14:21, Chris Torek nos...@torek.net wrote: | In article mailman.854.1315441399.27778.python-l...@python.org | Cameron Simpson c...@zip.com.au wrote: | Facilities like feof() in C and eof in Pascal already lead to lots of | code that runs happily with flat files and behaves badly in interactive | or piped input. It is _so_ easy to adopt a style like: | | while not eof(filehandle): | line = filehandle.nextline() | ... | | Minor but important point here: eof() in Pascal is predictive (uses | a crystal ball to peer into the future to see whether EOF is is | about to occur -- which really means, reads ahead, causing that | interactivity problem you mentioned), but feof() in C is post-dictive. | The feof(stream) function returns a false value if the stream has | not yet encountered an EOF, but your very next attempt to read from | it may (or may not) immediately encounter that EOF. Thanks. I had forgotten this nuance. Cheers, -- Cameron Simpson c...@zip.com.au DoD#743 http://www.cskk.ezoshosting.com/cs/ Where am I? In the Village. What do you want? Information. Whose side are you on? That would be telling. We want information. Information. Information! You won't get it! By hook or by crook, we will. Who are you? The new number 2. Who is number 1? You are number 6. I am not a number, I am a free man! [Laughter] -- http://mail.python.org/mailman/listinfo/python-list
Best way to check that you are at the beginning (the end) of an iterable?
Hi there, What is the simplest way to check that you are at the beginning or at the end of an iterable? I'm using enumerate with Python 3.2 (see below) but I'm wondering if there would be a better way. l = ['a', 'b', 'a', 'c'] for pos, i in enumerate(l): if pos == 0: print(head =, i) else: print(i) I know that Python is not exactly a functional language but wouldn't something like ishead() or istail() be useful? -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
On 07Sep2011 14:35, Laurent laurent.pa...@gmail.com wrote: | What is the simplest way to check that you are at the beginning or at | the end of an iterable? I'm using enumerate with Python 3.2 (see below) | but I'm wondering if there would be a better way. | | l = ['a', 'b', 'a', 'c'] | | for pos, i in enumerate(l): | if pos == 0: | print(head =, i) | else: | print(i) | | I know that Python is not exactly a functional language but wouldn't | something like ishead() or istail() be useful? There are a few reasons these do not exist out of the box (quite aside from how easy it is to do on the occasions you actually want it). Tackling ishead and istail in turn... The ishead() would need to be a top level function (like len()) because if it were an iterator method, every iterator would need to have it implemented; currently the number of methods needed to roll your own iterator is just two (iter and next). ishead() could be done as a top level function, though it would need the storage cost of an additional state value to every iterator (i.e. a first boolean or equivalent). So you'd be proposing more memory cost and possibly a retrospective code change for all the existing planetwide code, for a minor convenient. As you note, enumerate gets you a pos value, and it is easy enough to write a for loop like this: first = True for i in iterable_thing: if first: print head =, i else: print i first = False Your istail() is much worse. A generator would need to do lookahead to answer istail() in the general case. Consider iterating over the lines in a file, or better still the lines coming from a pipeline. Or iteraing over packets received on a network connection. You can't answer istail() there until you have seen the next line/packet (or EOF/connection close). And that may be an arbitrary amount of time in the future. You're going to stall your whole program for such a question? You can do this easily enough for yourself as an itertools-like thing: write a wrapper generator that answers ishead() and istail() for arbitrary iterators. Completely untested example code: class BoundSensitiveIterator(object): def __init__(self, subiter): self.sofar = 0 self.subiter = subiter self.pending = () def iter(self): return self def next(self): self.sofar += 1 if self.pending is None: raise StopIteration if self.pending: nxt = self.pending[0] self.pending = () return nxt return self.subiter.next() def ishead(self): # maybe = 1, depending on what you want it to mean return self.sofar == 1 def istail(self): if self.pending is None: return True if self.pending: return False try: nxt = self.subiter.next() except StopIteration: self.pending = None return True else: self.pending = (nxt,) return False I = BoundSensitiveIterator(other_iterable) for n in I: print n, ishead =, I.ishead(), istail =, I.istail() You can see it adds some performance and storage overhead, and of course may stall if you every ask istail() of an on demand iterable. About the only time I do this is my personal the() convenience function: def the(list, context=None): ''' Returns the first element of an iterable, but requires there to be exactly one. ''' icontext=expected exactly one value if context is not None: icontext=icontext+ for +context first=True for elem in list: if first: it=elem first=False else: raise IndexError, %s: got more than one element (%s, %s, ...) \ % (icontext, it, elem) if first: raise IndexError, %s: got no elements % icontext return it Which I use as a definite article in places where an iterable _should_ yield exactly one result (eg SQL SELECTs that _ought_ to get exactly one hit). I can see I wrote that a long time ago - it could do with some style fixes. And a code scan shows it sees little use:-) Cheers, -- Cameron Simpson c...@zip.com.au DoD#743 http://www.cskk.ezoshosting.com/cs/ Electronic cardboard blurs the line between printed objects and the virtual world. - overhead by WIRED at the Intelligent Printing conference Oct2006 -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
I totally understand the performance issue that an hypothetical istail would bring, even if I think it would just be the programmer's responsibility not to use it when it's not certain that an end can be detected. But I don't see why *adding* something like ishead would be so bad (at worse by using a boolean somewhere as you mentioned). Anyway I was just asking if there is something better than enumerate. So the answer is no? The fact that I have to create a tuple with an incrementing integer for something as simple as checking that I'm at the head just sounds awfully unpythonic to me. -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
On 07Sep2011 16:22, Laurent laurent.pa...@gmail.com wrote: | I totally understand the performance issue that an hypothetical | istail would bring, even if I think it would just be the programmer's | responsibility not to use it when it's not certain that an end can | be detected. The trouble with these things is that their presence leads to stallable code, often in libraries. Let the programmer write code dependent on istail() without thinking of the stall case (or even the gratuitous execution case, as in a generator with side effects in calling .next()) and have that buried in a utilities function. Facilities like feof() in C and eof in Pascal already lead to lots of code that runs happily with flat files and behaves badly in interactive or piped input. It is _so_ easy to adopt a style like: while not eof(filehandle): line = filehandle.nextline() ... that is it often thought that having offered the eof() function is a design error. (Of course in the example above the usual python idiom would win out from existing habit, but there are plenty of other situations where is would just be _easy_ to rely of istail() in whatever form.) | But I don't see why *adding* something like ishead would be so bad | (at worse by using a boolean somewhere as you mentioned). It is not awful, but as remarked: - extra storage cost to _every_ iterable, for a rarely used facility - extra runtime cost to maintain the state - _retroactive_ burden on _every_ iterator implementation presently existing; every iterator sudden needs to implement and offer this extra facility to be generate purpose use - it is easy to provide the facility on the rare occasions when it is needed Personally, I think point 3 above is the killer and 1 and 2 are serious counter arguments. | Anyway I was just asking if there is something better than enumerate. So | the answer is no? The fact that I have to create a tuple with an | incrementing integer for something as simple as checking that I'm at | the head just sounds awfully unpythonic to me. You can just use a boolean if you like. I have plent of loops like: first = true for i in iterable: if first: blah ... ... first = False Cheap and easy. Cheers, -- Cameron Simpson c...@zip.com.au DoD#743 http://www.cskk.ezoshosting.com/cs/ Bye and bye, God caught his eye, - Epitaph for a waiter by David McCord -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
Laurent wrote: Hi there, What is the simplest way to check that you are at the beginning or at the end of an iterable? I don't think this question is meaningful. There are basically two fundamental types of iterables, sequences and iterators. Sequences have random access and a length, so if the start and end of the sequence is important to you, just use indexing: beginning = sequence[0] end = sequence[-1] for i, x in enumerate(sequence): if i == 0: print(at the beginning) elif i == len(sequence)-1: print(at the end) print(x) Iterators don't have random access, and in general they don't have a beginning or an end. There may not be any internal sequence to speak of: the iterator might be getting data from a hardware device that provides values continuously, or some other series of values without a well-defined beginning or end. Example: def time(): from time import asctime while True: yield asctime() it = time() What would it even mean to say that I am at the beginning or end of it? Iterators have no memory, so in one sense you are *always* at the beginning of the iterator: next() always returns the next item, and the previous item is lost forever. So the answer to the question Am I at the beginning of an iterator? is always You are now. For sequences, the question is best handled differently. For iterators, the question doesn't make sense in general. If you need an iterator that can report its internal state, write your own: import random, time class MyIter(object): def __init__(self): self.start = True self.end = False def __next__(self): if self.start: self.start = False if self.end: raise StopIteration if random.random() 0.01: self.end = True return time.asctime() def __iter__(self): return self -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
I guess enumerate is the best way to check for first argument. Note that if someone passes you the iterator as argument you have now way of checking if the consumed items from it. istail can be implemented using itertools.chain, see https://gist.github.com/1202260 -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
Yes of course the use of a boolean variable is obvious but I'm mixing python code with html using Mako templates. In Mako for code readability reasons I try to stick to simple for and if constructions, and I try to avoid variables declarations inside the html, that's all. Thanks anyway. -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
On 09/07/11 18:22, Laurent wrote: Anyway I was just asking if there is something better than enumerate. So the answer is no? The fact that I have to create a tuple with an incrementing integer for something as simple as checking that I'm at the head just sounds awfully unpythonic to me. I've made various generators that are roughly (modulo edge-condition error checking) something like def with_prev(it): prev = None for i in it: yield prev, i i = prev def with_next(it): prev = it.next() for i in it: yield prev, i prev = i yield prev, None which can then be used something like your original for cur, next in with_next(iterable): if next is None: do_something_with_last(cur) else: do_regular_stuff_with_non_last(cur) for prev, cur in with_prev(iterable): if prev is None: do_something_with_first(cur) else: do_something_with_others(cur) If your iterable can return None, you could create a custom object to signal the non-condition: NO_ITEM = object() and then use NO_ITEM in place of None in the above code. -tkc -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
On 9/7/2011 8:23 PM, Cameron Simpson wrote: On 07Sep2011 16:22, Laurentlaurent.pa...@gmail.com wrote: | I totally understand the performance issue that an hypothetical | istail would bring, even if I think it would just be the programmer's | responsibility not to use it when it's not certain that an end can | be detected. The trouble with these things is that their presence leads to stallable code, often in libraries. Let the programmer write code dependent on istail() without thinking of the stall case (or even the gratuitous execution case, as in a generator with side effects in calling .next()) and have that buried in a utilities function. Facilities like feof() in C and eof in Pascal already lead to lots of code that runs happily with flat files and behaves badly in interactive or piped input. It is _so_ easy to adopt a style like: while not eof(filehandle): line = filehandle.nextline() ... that is it often thought that having offered the eof() function is a design error. (Of course in the example above the usual python idiom would win out from existing habit, but there are plenty of other situations where is would just be _easy_ to rely of istail() in whatever form.) | But I don't see why *adding* something like ishead would be so bad | (at worse by using a boolean somewhere as you mentioned). It is not awful, but as remarked: - extra storage cost to _every_ iterable, for a rarely used facility - extra runtime cost to maintain the state - _retroactive_ burden on _every_ iterator implementation presently existing; every iterator sudden needs to implement and offer this extra facility to be generate purpose use - it is easy to provide the facility on the rare occasions when it is needed Personally, I think point 3 above is the killer and 1 and 2 are serious counter arguments. The iterator protocol is intentionally as simple as sensibly possible. | Anyway I was just asking if there is something better than enumerate. So | the answer is no? The fact that I have to create a tuple with an | incrementing integer for something as simple as checking that I'm at | the head just sounds awfully unpythonic to me. You can just use a boolean if you like. I have plent of loops like: first = true for i in iterable: if first: blah ... ... first = False Cheap and easy. Cheers, Or grab and process the first item separately from the rest. it = iter(iterable) try: first = next(it) process first item except StopIteration: raise ValueError(Empty iterable not allowed) for i in it: process non-first item -- Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
Yes, I was just hoping for something already included that I wouldn't know (i'm new to Python). -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
Interesting. I will check that yield functionality out. Thanks. -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
On 9/7/2011 8:24 PM, Steven D'Aprano wrote: I don't think this question is meaningful. There are basically two fundamental types of iterables, sequences and iterators. And non-sequence iterables like set and dict. Sequences have random access and a length, so if the start and end of the sequence is important to you, just use indexing: beginning = sequence[0] end = sequence[-1] for i, x in enumerate(sequence): if i == 0: print(at the beginning) elif i == len(sequence)-1: print(at the end) print(x) And finite non-sequences can be turned into sequences with list(iterable). -- Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
I don't think this question is meaningful. There are basically two fundamental types of iterables, sequences and iterators. Sequences have random access and a length, so if the start and end of the sequence is important to you, just use indexing: beginning = sequence[0] end = sequence[-1] for i, x in enumerate(sequence): if i == 0: print(at the beginning) elif i == len(sequence)-1: print(at the end) print(x) Iterators don't have random access, and in general they don't have a beginning or an end. There may not be any internal sequence to speak of: the iterator might be getting data from a hardware device that provides values continuously, or some other series of values without a well-defined beginning or end. Maybe I should have said best way to check that you didn't start the iteration process yet but you see what I mean. Well I guess I have to unlearn my bad lisp/scheme habits... -- http://mail.python.org/mailman/listinfo/python-list
Re: Best way to check that you are at the beginning (the end) of an iterable?
On Wed, Sep 7, 2011 at 5:24 PM, Miki Tebeka miki.teb...@gmail.com wrote: I guess enumerate is the best way to check for first argument. Note that if someone passes you the iterator as argument you have now way of checking if the consumed items from it. istail can be implemented using itertools.chain, see https://gist.github.com/1202260 For the archives, if Gist ever goes down: from itertools import chain def istail(it): '''Check if iterator has one more element. Return True/False and iterator.''' try: i = next(it) except StopIteration: return False, it try: j = next(it) return False, chain([i, j], it) except StopIteration: return True, chain([i], it) t, it = istail(iter([])) print t, list(it) t, it = istail(iter([1])) print t, list(it) t, it = istail(iter([1, 2])) print t, list(it) -- http://mail.python.org/mailman/listinfo/python-list
