idioms

Over the years, "regex" pattern matching has been getting more and more powerful to the point where I wonder: is it really just context-sensitive-grammar matching? Is it a variation/extension of context-free-grammar matching? Where is it right now and why don't we just call it that instead of the old, restrictive "regular expression"? In particular backreferences to capturing parentheses make regular expressions more complex than regular, context-free, or context-sensitive grammars. The name is simply historically grown (as many words). See also this section in Wikipedia and this explanation

This question already has an answer here: Underscore _ as variable name in Python [duplicate] 3 answers I have seen this in a few contexts, e.g., in sequence unpacking: _, x = L.pop() # e.g., L is a list of tuples to initialize a container: X = _ So obviously this is not an element of the formal python syntax, rather the uses i am aware of appear discretionary. So I'm curious what is the likely reason for its use and what are the advantages generally (if any)? Note : my question relates to the use of "_" in scripts, modules, etc., but not its use at the interactive prompt. In IDLE , the

The "goatse operator" or the =()= idiom in Perl causes an expression to be evaluated in list context. An example is: my $str = "5 and 4 and a 3 and 2 1 BLAST OFF!!!"; my $count =()= $str =~ /\d/g; # 5 matches... print "There are $count numbers in your countdown...\n\n"; As I interprete the use, this is what happens: $str =~ /\d/g matches all the digits. The g switch and list context produces a list of those matches. Let this be the "List Producer" example, and in Perl this could be many things. the =()= causes an assignment to an empty list, so all the actual matches are copied to an empty

If I have a function with multiple conditional statements where every branch gets executed returns from the function. Should I use multiple if statements, or if/elif/else? For example, say I have a function: def example(x): if x > 0: return 'positive' if x < 0: return 'negative' return 'zero' Is it better to write: def example(x): if x > 0: return 'positive' elif x < 0: return 'negative' else: return 'zero' Both have the same outcome, but is one more efficient or considered more idiomatic than the other? Edit: A couple of people have said that in the first example both if statements are always

This question already has an answer here: How to merge two dictionaries in a single expression? 41 answers How do you calculate the union of two dict objects in Python, where a (key, value) pair is present in the result iff key is in either dict (unless there are duplicates)? For example, the union of {'a' : 0, 'b' : 1} and {'c' : 2} is {'a' : 0, 'b' : 1, 'c' : 2} . Preferably you can do this without modifying either input dict . Example of where this is useful: Get a dict of all variables currently in scope and their values Mechanical snail This question provides an idiom. You use one of the

I am looking for a concise way to check a value to see if it is nil or zero. Currently I am doing something like: if (!val || val == 0) # Is nil or zero end But this seems very clumsy. Christian Lescuyer Objects have a nil? method . if val.nil? || val == 0 [do something] end Or, for just one instruction: [do something] if val.nil? || val == 0 If you really like method names with question marks at the end: if val.nil? || val.zero? # do stuff end Your solution is fine, as are a few of the other solutions. Ruby can make you search for a pretty way to do everything, if you're not careful. First

Creating hashes of hashes in Ruby allows for convenient two (or more) dimensional lookups. However, when inserting one must always check if the first index already exists in the hash. For example: h = Hash.new h['x'] = Hash.new if not h.key?('x') h['x']['y'] = value_to_insert It would be preferable to do the following where the new Hash is created automatically: h = Hash.new h['x']['y'] = value_to_insert Similarly, when looking up a value where the first index doesn't already exist, it would be preferable if nil is returned rather than receiving an undefined method for '[]' error. looked_up