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Clojure-like utilities

Project description


clj is a collection of functions to work with lazy iterators in Python.

It’s a module for those times when you did too much Clojure and came back to Python wondering where are all these distinct, drop-while, cycle, first, etc.


pip install clj



;; Clojure
(println (count (distinct (filter even? (map inc coll)))))
# Python
from clj import count, distinct, filter, inc, map

even = lambda e: ~e&1

print(count(distinct(filter(even, map(inc, coll)))))

Note that count() works on both sequences in generators; in the latter case it doesn’t load everything in memory like e.g. len(list(g)) would do.

Core Ideas

  • Lazy by default. All functions should work on arbitrary iterators and return generators.
  • This is Python. We keep Python’s semantics instead of trying to reproduce Clojure in Python (e.g. 0 and [] are logically true in Clojure but false in Python; None is not equivalent to an empty collection).
  • Don’t Reinvent the Wheel. We don’t reimplement built-in functions unless they miss something: range because it can’t be called without argument to yield an infinite sequence; map and filter in Python 2 because they aren’t lazy.


The general naming scheme is: use underscores instead of hyphens; start the function with is_ if its Clojure counterparts ends with a ?.


We aim to implement all Clojure functions that operate on sequences (see the list here). They all work on iterables and return generators by default (Python’s closest equivalent of lazy seqs). We don’t support transducers.

Clojure clj Comment
distinct distinct
filter filter
remove remove
keep keep
keep-indexed keep_indexed 
cons cons
concat concat Deprecated. Use Python’s itertools.chain
lazy-cat - Use Python’s itertools.chain
mapcat mapcat
cycle cycle
interleave interleave
interpose interpose
rest rest
next - Use rest.
fnext - Use second.
nnext - Use rest(rest(…))
drop drop
drop-while drop_while Deprecated. Use Python’s itertools.dropwhile
nthnext - Use drop.
take take
take-nth take_nth
take-while take_while Deprecated. Use Python’s itertools.takewhile
butlast butlast
drop-last drop_last
flatten flatten
reverse reverse
sort - Use Python’s built-in sort.
sort-by - Use sort(…, key=your_function).
shuffle shuffle
split-at split_at
split-with split_with
map map
pmap -
replace replace
reductions reductions (reductions f i c) becomes reductions(f, c, i)
map-indexed map_indexed
seque -
first first 
ffirst ffirst
nfirst nfirst
second second
nth nth
last last
rand-nth - Use Python’s random.choice.
zipmap zipmap
into -
reduce - Use Python’s built-in reduce.
set - Use Python’s set.
vec - Use Python’s list.
into-array - Use Python’s list.
to-array-2d -
frequencies - Use Python’s collections.Counter.
group-by group_by
apply - Use the f(*args) construct.
not-empty -
some some
seq? is_seq
every? every
not-every? not_every
not-any? not_any
empty? -
empty empty
doseq - Use for … in.
dorun dorun
doall - Use Python’s list.
realized? -
seq -
vals - Use Python’s dict.values.
keys - Use Python’s dict.keys.
rseq -
repeatedly repeatedly
iterate iterate
repeat repeat (repeat n x) becomes repeat(x, n).
range range Prefer Python’s range for everything but infinite generators.
line-seq - Loop over an io.BufferedReader.
resultset-seq -
re-seq - Use Python’s re.finditer.
tree-seq tree_seq
file-seq - Use Python’s os.walk.
xml-seq -
iterator-seq -
enumeration-seq -
hash-map - Use Python’s dict.
array-map - Use Python’s dict.
sorted-map - Use collections.OrderedDict.
hash-set - Use Python’s set.
set - Use Python’s set.

We also implemented count, which uses Python’s len when possible and fallbacks on a for loop for other cases.


We also provide miscellaneous functions as well as functions that work on functions.

Clojure clj Comment
identity identity
partial - Use Python’s functools.partial
comp comp
complement complement
constantly constantly
juxt juxt
distinct? is_distinct
Clojure clj Comment
inc inc
dec dec

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