raymondhによるPython tips

Raymond Hettinger @raymondh

#python tip: The set.discard(k) method unconditionally removes k from a set. For dictionaries, use dict.pop(k, None).

Raymond Hettinger @raymondh

#python tip: The fast itertools recipes can be copied to a utils module with a single cut-and-paste. http://bit.ly/itertools_plus

Raymond Hettinger @raymondh

#python tip: Turn an argparse namespace into a regular dictionary by running vars() on the parse_args() result

Raymond Hettinger @raymondh

#python tip: bytearray(1000) initializes a 1000 byte mutuale array with all values set to zero.

Raymond Hettinger @raymondh

#python tip: define __instancecheck__ to let a class use new-style duck typing to fool isinstance() http://bit.ly/override_isinstance

Raymond Hettinger @raymondh

#python tip: use named tuples liberally, almost never index a tuple, either unpack it or use attribute access.

Brian K. Jones @bkjones

#python tip: namedtuple instances can be passed as '*args', and can be used a la '{0.name}, {0.value}'.format(myobj).

Raymond Hettinger @raymondh

#python pro tip: Slices with negative steps are hard to grok. Use reversed() instead. s[j-1:i-1:-1] --> list(reversed(s[i:j]))

Raymond Hettinger @raymondh

#python tip: the % modulo operation in #python differs from some other languages in that a%b will have the same sign as b.

Raymond Hettinger @raymondh

#python tip: operator.iadd() will add but can't assign: t=(1,2); iadd(t, (3,)) returns (1,2,3) but t is unchanged!

Raymond Hettinger @raymondh

#python note: operator.iadd() can appear to make assignments: t=[1,2]; iadd(t, [3]) --> [1,2,3] and t will mutate.

Raymond Hettinger @raymondh

#python tip: Replace unintelligible unicode hex literals with names: int(u'\N{arabic-indic digit four}\N{arabic-indic digit two}')

Raymond Hettinger @raymondh

#python tip: though functions like sort() and bisect() only call __lt__(), PEP 8 advises you always define all 6 rich comparisons

Raymond Hettinger @raymondh

#python tip: IDLE's file menu has "open module" and "class browser" which are great for finding and getting an overview of code.

Raymond Hettinger @raymondh

#python tip: Decimal's unary plus is more useful than it looks. It applies the current context (rounding, limits, etc).

Raymond Hettinger @raymondh

#python tip: Decimal objects are exact numbers that are independent of context (you can have a 50 digit number in a 20 digit context).

Raymond Hettinger @raymondh

#python tip: Decimal contexts are applied after or during math operations, not before.

Raymond Hettinger @raymondh

#python tip: c += Counter() will eliminate all entries with non-positive counts.

Raymond Hettinger @raymondh

#python tip: When using __slots__, you can add entries for __weakref__ and __dict__ as needed.

Raymond Hettinger @raymondh

#python tip: some_sorted_dict = OrderedDict(sorted(somedict.items())) # note: need to re-sort if new keys are added

Raymond Hettinger @raymondh

#python tip: One way to implement the observer design pattern: http://t.co/74CMd6M

Raymond Hettinger @raymondh

#python tip: collections.deque() takes an optional maxlen argument to automatically pop old elements when news ones are added.

Raymond Hettinger @raymondh

#python tip: here's a fast, universal constant function to use with collections.defaultdict: f=itertools.repeat(someconst).next

Raymond Hettinger @raymondh

#python tip: Every time you add __getitem__() indexing to a class, think about adding slicing support too.

Raymond Hettinger @raymondh

#python tip: whenever a slice has negative variable for the right index, such as s[:-i], I suspect a bug for the case where i==0