Functions

identity(x)

Returns its argument.

constantly(x)

Returns function accepting any args, but always returning x.

caller(*args, **kwargs)

Returns function calling its argument with passed arguments.

partial(func, *args, **kwargs)

Returns partial application of func. A re-export of functools.partial(). Can be used in a variety of ways. DSLs is one of them:

field = dict
json_field = partial(field, json=True)
rpartial(func, *args)

Partially applies last arguments in func:

from operator import div
one_third = rpartial(div, 3.0)

Arguments are passed to func in the same order as they came to rpartial():

separate_a_word = rpartial(str.split, ' ', 1)
func_partial(func, *args, **kwargs)

Like partial() but returns a real function. Which is useful when, for example, you want to create a method of it:

setattr(self, 'get_%s_display' % field.name,
        func_partial(_get_FIELD_display, field))

Use partial() if you are ok to get callable object instead of function as it’s faster.

curry(func[, n])

Curries function. For example, given function of two arguments f(a, b) returns function:

lambda a: lambda b: f(a, b)

Handy to make a partial factory:

make_tester = curry(re_test)
is_word = make_tester(r'^\w+$')
is_int = make_tester(r'^[1-9]\d*$')

But see re_tester() if you need this particular one.

rcurry(func[, n])

Curries function from last argument to first:

has_suffix = rcurry(str.endswith)
lfilter(has_suffix("ce"), ["nice", "cold", "ice"])
# -> ["nice", "ice"]

Can fix number of arguments when it’s ambiguous:

to_power = rcurry(pow, 2) # curry 2 first args in reverse order
to_square = to_power(2)
to_cube = to_power(3)
autocurry(func)

Constructs a version of func returning its partial applications until sufficient arguments are passed:

def remainder(what, by):
    return what % by
rem = autocurry(remainder)

assert rem(10, 3) == rem(10)(3) == rem()(10, 3) == 1
assert map(rem(by=3), range(5)) == [0, 1, 2, 0, 1]

Can clean your code a bit when partial() makes it too cluttered.

compose(*fs)

Returns composition of functions:

extract_int = compose(int, r'\d+')

Supports Extended function semantics.

rcompose(*fs)

Returns composition of functions, with functions called from left to right. Designed to facilitate transducer-like pipelines:

# Note the use of iterator function variants everywhere
process = rcompose(
    partial(remove, is_useless),
    partial(map, process_row),
    partial(chunks, 100)
)

for chunk in process(data):
    write_chunk_to_db(chunk)

Supports Extended function semantics.

juxt(*fs)
ljuxt(*fs)

Takes several functions and returns a new function that is the juxtaposition of those. The resulting function takes a variable number of arguments, and returns an iterator or a list containing the result of applying each function to the arguments.

iffy([pred, ]action[, default=identity])

Returns function, which conditionally, depending on pred, applies action or default. If default is not callable then it is returned as is from resulting function. E.g. this will call all callable values leaving rest of them as is:

map(iffy(callable, caller()), values)

Common use it to deal with messy data:

dirty_data = ['hello', None, 'bye']
lmap(iffy(len), dirty_data)              # => [5, None, 3]
lmap(iffy(isa(str), len, 0), dirty_data) # => [5, 0, 3], also safer

See also silent() for easier use cases.

Function logic

This family of functions supports creating predicates from other predicates and regular expressions.

complement(pred)

Constructs a negation of pred, i.e. a function returning a boolean opposite of original function:

is_private = re_tester(r'^_')
is_public = complement(is_private)

# or just
is_public = complement(r'^_')
all_fn(*fs)
any_fn(*fs)
none_fn(*fs)
one_fn(*fs)

Construct a predicate returning True when all, any, none or exactly one of fs return True. Support short-circuit behavior.

is_even_int = all_fn(isa(int), even)
some_fn(*fs)

Constructs function calling fs one by one and returning first true result.

Enables creating functions by short-circuiting several behaviours:

get_amount = some_fn(
    lambda s: 4 if 'set of' in s else None,
    r'(\d+) wheels?',
    compose({'one': 1, 'two': 2, 'pair': 2}, r'(\w+) wheels?')
)

If you wonder how on Earth one can compose() dict and string see Extended function semantics.

count(start=0, step=1)

Makes infinite iterator of values:
start, start + step, start + 2*step, ...
cycle(seq)

Cycles passed sequence indefinitely
yielding its elements one by one.
repeat(item[, n])

Makes an iterator yielding item for n times
or indefinitely if n is omitted.
repeatedly(f[, n])

Takes a function of no args, presumably with side effects,
and returns an infinite (or length n) iterator of calls to it.
iterate(f, x)

Returns an infinite iterator of x, f(x), f(f(x)), ...
re_all(regex, s, flags=0)

Lists all matches of regex in s.
re_iter(regex, s, flags=0)

Iterates over matches of regex in s.
first(seq)

Returns the first item in the sequence.
Returns None if the sequence is empty.
second(seq)

Returns second item in the sequence.
Returns None if there are less than two items in it.
last(seq)

Returns the last item in the sequence.
Returns None if the sequence is empty.
nth(seq)

Returns nth item in the sequence
or None if no such item exists.
some([pred, ]seq)

Finds first item in seq passing pred
or first that is true if pred is omitted.
take(n, seq)

Returns a list of first n items in the sequence,
or all items if there are fewer than n.
drop(n, seq)

Skips first n items in the sequence,
yields the rest.
rest(seq)

Skips first item in the sequence, yields the rest.
butlast(seq)

Yields all elements of the sequence but last.
takewhile([pred, ]seq)

Yields seq items as long as they pass pred.
dropwhile([pred, ]seq)

Skips elements of seq while pred passes
and then yields the rest.
split_at(n, seq)
lsplit_at(n, seq)


Splits the sequence at given position,
returning a tuple of its start and tail.
split_by(pred, seq)
lsplit_by(pred, seq)


Splits the start of the sequence,
consisting of items passing pred,
from the rest of it.
map(f, *seqs)
lmap(f, *seqs)


Extended versions of map() and list(map())
mapcat(f, *seqs)
lmapcat(f, *seqs)


Maps given sequence(s) and concatenates the results.
keep([f, ]*seqs)
lkeep([f, ]*seqs)


Maps seq with f and filters out falsy results.
Simply removes falsy values in one argument version.
pluck(key, mappings)
lpluck(key, mappings)


Yields or lists values for key in each mapping.
pluck_attr(attr, objects)
lpluck_attr(attr, objects)


Yields or lists values of attr of each object.
invoke(objects, name, *args, **kwargs)
linvoke(objects, name, *args, **kwargs)


Yields or lists results of the given method call
for each object in objects.
@wrap_prop(ctx)

Wrap a property accessors with ctx.
filter(pred, seq)
lfilter(pred, seq)


Extended versions of filter() and list(filter()).
remove(pred, seq)
lremove(pred, seq)


Removes items from seq passing given predicate.
distinct(seq, key=identity)
ldistinct(seq, key=identity)


Removes items having same key from seq.
Preserves order.
where(mappings, **cond)
lwhere(mappings, **cond)


Selects mappings containing all pairs in cond.
without(seq, *items)
lwithout(seq, *items)


Returns sequence without items,
preserves order.
cat(seqs)
lcat(seqs)


Concatenates passed sequences.
concat(*seqs)
lconcat(*seqs)


Concatenates several sequences.
flatten(seq, follow=is_seqcont)
lflatten(seq, follow=is_seqcont)


Flattens arbitrary nested sequence,
dives into when follow(item) is truthy.
interleave(*seqs)

Yields first item of each sequence,
then second one and so on.
interpose(sep, seq)

Yields items of seq separated by sep.
lzip(*seqs)

List version of zip()
chunks(n, [step, ]seq)
lchunks(n, [step, ]seq)


Chunks seq into parts of length n or less.
Skips step items between chunks.
partition(n, [step, ]seq)
lpartition(n, [step, ]seq)


Partitions seq into parts of length n.
Skips step items between parts.
Non-fitting tail is ignored.
partition_by(f, seq)
lpartition_by(f, seq)


Partition seq into continuous chunks
with constant value of f.
split(pred, seq)
lsplit(pred, seq)


Splits seq items which pass pred
from the ones that don't.
count_by(f, seq)

Counts numbers of occurrences of values of f
on elements of seq.
count_reps(seq)

Counts repetitions of each value in seq.
group_by(f, seq)

Groups items of seq by f(item).
group_by_keys(get_keys, seq)

Groups elements of seq by multiple keys.
group_values(seq)

Groups values of (key, value) pairs by keys.
ilen(seq)

Consumes the given iterator and returns its length.
reductions(f, seq[, acc])
lreductions(f, seq[, acc])


Constructs intermediate reductions of seq by f.
sums(seq[, acc])
lsums(seq[, acc])


Returns a sequence of partial sums of seq.
all([pred, ]seq)

Checks if all items in seq pass pred.
any([pred, ]seq)

Checks if any item in seq passes pred.
none([pred, ]seq)

Checks if none of the items in seq pass pred.
one([pred, ]seq)

Checks if exactly one item in seq passes pred.
pairwise(seq)

Yields all pairs of neighboring items in seq.
with_prev(seq, fill=None)

Yields each item from seq with the one preceding it.
with_next(seq, fill=None)

Yields each item from seq with the next one.
zip_values(*dicts)

Yields tuples of corresponding values of given dicts.
zip_dicts(*dicts)

Yields tuples like (key, val1, val2, ...)
for each common key in all given dicts.
tree_leaves(root, follow=is_seqcont, children=iter)
ltree_leaves(root, follow=is_seqcont, children=iter)


Lists or iterates over tree leaves.
tree_nodes(root, follow=is_seqcont, children=iter)
ltree_nodes(root, follow=is_seqcont, children=iter)


Lists or iterates over tree nodes.
merge(*colls)

Merges several collections of same type into one:
dicts, sets, lists, tuples, iterators or strings
For dicts later values take precedence.
merge_with(f, *dicts)

Merges several dicts combining values with given function.
join(colls)

Joins several collections of same type into one.
Same as merge() but accepts sequence of collections.
join_with(f, *dicts)

Joins several dicts combining values with given function.
walk(f, coll)

Maps coll with f, but preserves collection type.
walk_keys(f, coll)

Walks keys of coll, mapping them with f.
Works with dicts and collections of pairs.
walk_values(f, coll)

Walks values of coll, mapping them with f.
Works with dicts and collections of pairs.
select(pred, coll)

Filters elements of coll by pred
constructing a collection of same type.
select_keys(pred, coll)

Select part of coll with keys passing pred.
Works with dicts and collections of pairs.
select_values(pred, coll)

Select part of coll with values passing pred.
Works with dicts and collections of pairs.
compact(coll)

Removes falsy values from given collection.
All collections functions work with dicts.
These are targeted specifically at them.
flip(mapping)

Flip passed dict swapping its keys and values.
zipdict(keys, vals)

Creates a dict with keys mapped to the corresponding vals.
itervalues(coll)

Yields values of the given collection.
iteritems(coll)

Yields (key, value) pairs of the given collection.
project(mapping, keys)

Leaves only given keys in mapping.
omit(mapping, keys)

Removes given keys from mapping.
empty(coll)

Returns an empty collection of the same type as coll.
get_in(coll, path, default=None)

Returns a value at path in the given nested collection.
set_in(coll, path, value)

Creates a copy of coll with the value set at path.
update_in(coll, path, update, default=None)

Creates a copy of coll with a value updated at path.
del_in(coll, path)

Creates a copy of coll with path removed.
has_path(coll, path)

Tests whether path exists in a nested coll.
Most of functions in this section support extended semantics.
identity(x)

Returns its argument.
constantly(x)

Creates a function accepting any args, but always returning x.
func_partial(func, *args, **kwargs)

Like partial() but returns a real function.
partial(func, *args, **kwargs)

Returns partial application of func.
rpartial(func, *args)

Partially applies last arguments to func.
iffy([pred, ]action[, default=identity])

Creates a function, which conditionally applies action or default.
caller(*args, **kwargs)

Creates a function calling its argument with passed arguments.
re_finder(regex, flags=0)

Creates a function finding regex in passed string.
re_tester(regex, flags=0)

Creates a predicate testing passed strings with regex.
complement(pred)

Constructs a complementary predicate.
autocurry(func)

Creates a version of func returning its partial applications
until sufficient arguments are passed.
curry(func[, n])

Curries func into a chain of one argument functions.
Arguments are passed from left to right.
rcurry(func[, n])

Curries func from right to left.
compose(*fs)

Composes passed functions.
rcompose(*fs)

Composes fs, calling them from left to right.
juxt(*fs)
ljuxt(*fs)


Constructs a juxtaposition of the given functions.
Result returns a list or an iterator of results of fs.
all_fn(*fs)

Constructs a predicate,
which holds when all fs hold.
any_fn(*fs)

Constructs a predicate,
which holds when any of fs holds.
none_fn(*fs)

Constructs a predicate,
which holds when none of fs hold.
one_fn(*fs)

Constructs a predicate,
which holds when exactly one of fs holds.
some_fn(*fs)

Constructs a function, which calls fs one by one
and returns first truthy result.
is_distinct(*fs)

Checks if all elements in the collection are different.
isa(*types)

Creates a function checking if its argument
is of any of given types.
is_iter(value)

Checks whether value is an iterator.
is_mapping(value)

Checks whether value is a mapping.
is_set(value)

Checks whether value is a set.
is_list(value)

Checks whether value is a list.
is_tuple(value)

Checks whether value is a tuple.
is_seq(value)

Checks whether value is a Sequence.
is_mapping(value)

Checks whether value is a mapping.
is_seqcoll(value)

Checks whether value is a list or a tuple,
which are both sequences and collections.
is_seqcont(value)

Checks whether value is a list, a tuple or an iterator,
which are both sequences and containers.
iterable(value)

Checks whether value is iterable.
@decorator

Transforms a flat wrapper into a decorator.
@wraps

An utility to pass function metadata
from wrapped function to a wrapper.
unwrap(func)

Get the object wrapped by func.
@once

Let function execute only once.
Noop all subsequent calls.
@once_per(*argnames)

Call function only once for every combination
of the given arguments.
@once_per_args

Call function only once for every combination
of values of its arguments.
@collecting

Transforms a generator into list returning function.
@joining(sep)

Joins decorated function results with sep.
@post_processing(func)

Post processes decorated function result with func.
@throttle(period)

Only runs a decorated function once per period.
@wrap_with(ctx)

Turn context manager into a decorator.
nullcontext(enter_result=None)

A noop context manager.
@retry(tries, errors=Exception, timeout=0, filter_errors=None)

Tries decorated function up to tries times.
Retries only on specified errors.
@silent

Alters function to ignore all exceptions.
@ignore(errors, default=None)

Alters function to ignore errors,
returning default instead.
suppress(*errors)

The context manager suppressing errors in its block.
@limit_error_rate(fails, timeout, ...)

If function fails to complete fails times in a row,
calls to it will be blocked for timeout seconds.
fallback(*approaches)

Tries several approaches until one works.
Each approach has a form of (callable, errors).
raiser(exception=Exception, *args, **kwargs)

Constructs function that raises the given exception
with given arguments on any invocation.
@reraise(errors, into)

Intercepts errors and reraises them as into exception.
tap(x, label=None)

Prints x and then returns it.
@log_calls(print_func, errors=True, stack=True)
@print_calls(errors=True, stack=True)


Logs or prints all function calls,
including arguments, results and raised exceptions.
@log_enters(print_func)
@print_enters


Logs or prints on each enter to a function.
@log_exits(print_func, errors=True, stack=True)
@print_exits(errors=True, stack=True)


Logs or prints on each exit from a function.
@log_errors(print_func, label=None, stack=True)
@print_errors(label=None, stack=True)


Logs or prints all errors within a function or block.
@log_durations(print_func, label=None)
@print_durations(label=None)


Times each function call or block execution.
log_iter_durations(seq, print_func, label=None)
print_iter_durations(seq, label=None)


Times processing of each item in seq.
@memoize

Memoizes a decorated function results.
@cache(timeout)

Caches a function results for timeout seconds.
@cached_property

Creates a property caching its result.
@cached_property

Creates a read-only property caching its result.
@make_lookuper

Creates a cached function with prefilled memory.
@silent_lookuper

Creates a cached function with prefilled memory.
Ignores memory misses, returning None.
re_find(regex, s, flags=0)

Matches regex against the given string,
returns the match in the simplest possible form.
re_test(regex, s, flags=0)

Tests whether regex matches against s.
cut_prefix(s, prefix)

Cuts prefix from given string if it's present.
cut_suffix(s, suffix)

Cuts suffix from given string if it's present.
str_join([sep="", ]seq)

Joins the given sequence with sep.
Forces stringification of seq items.
@monkey(cls_or_module, name=None)

Monkey-patches class or module.
class namespace

A base class that prevents functions turning into methods.
class LazyObject(init)

Creates an object setting itself up on first use.
isnone(x)

Checks if x is None.
notnone(x)

Checks if x is not None.
inc(x)

Increments its argument by 1.
dec(x)

Decrements its argument by 1.
even(x)

Checks if x is even.
odd(x)

Checks if x is odd.