A simple syntax for piping in Python.

Compare:

from bookends import _
from toolz.curried import map

l = _| [3, 2, 1] | map(lambda n: n*2) | sorted |_   # [2, 4, 6]

with:

l = sorted(map(lambda n: n*2, [3, 2, 1]))

l = sorted([n*2 for n in [3, 2, 1]])

l = []
for n in [3, 2, 1]:
    l.append(n*2)
l.sort()

For an extended comparison, see example.py.

To install:

pip install bookends

To use:

from bookends import _

For similar tools, see:

• from fn import F
• from toolz import thread_first, thread_last, pipe
• Clojure's -> and ->>
• Underscore's chain
• Unix |

Note: for multiline usage, wrap the expression in parens.

import csv
from StringIO import StringIO

(_| '40,5,10\n20,6,9\n41,10,10\n'
  | StringIO
  | csv.reader
  | sorted
  |_)

# [['20', '6', '9'], ['40', '5', '10'], ['41', '10', '10']]

Wrap lone lambdas in parens as well.

(_| ['addition', 'multiplication']
  | (lambda l: l + ['exponentiation', 'tetration'])
  | ', '.join
  |_)

# 'addition, multiplication, exponentiation, tetration'

You can use partial or curried functions.

from functools import partial
from toolz.curried import drop

(_| ['ca', 'tx', 'ny']
  | partial(map, lambda state: state.upper())
  | drop(1)
  | list
  |_)

# ['TX', 'NY']

And/or use threading syntax, by putting each function and its arguments into a tuple.

from toolz import drop

(_| ['ca', 'tx', 'ny']
  | (map, lambda state: state.upper())
  | (drop, 1)
  | list
  |_)

# ['TX', 'NY']

If you don't like the underscore, import the bookend as B.

from bookends import B

(B| ['ca', 'tx', 'ny']
  | (map, lambda state: state.upper())
  | (drop, 1)
  | list
  |B)

To stop in the debugger before each function call, put a step into the pipe.

from bookends import step

(_| [3, 2, 1]
  | (map, lambda x: x*2)
  | step               # <==
  | sorted
  | sum
  |_)

To call off the stepping, drop in an endstep.

from bookends import step, endstep

(_| [3, 2, 1]
  | (map, lambda x: x*2)
  | step               # <==
  | sorted
  | endstep            # <==
  | sum
  |_)

To print each function and its output, drop in a verbose.

from bookends import verbose

(_| [3, 2, 1]
  | verbose            # <==
  | (map, lambda x: x*2)
  | sorted
  | sum
  |_)

You can easily add these options while debugging by tacking on their first letter to the initial bookend.

(_.sv| [3, 2, 1]       # <== Turn on step and verbose (_.s, _.v, and _.vs work too).
  | (map, lambda x: x*2)
  | sorted
  | sum
  |_)

Drop in a function that won't affect the operand by decorating it with passthrough.

from bookends import passthrough

@passthrough
def log(operand):
  log.info('Operand was {}.'.format(operand))

(_| [3, 2, 1]
  | (map, lambda x: x*2)
  | log                # <==
  | sorted
  |_)

Plays nice with Kachayev's _.

from fn import _ as __

_| [1, 2, 3] | __ + [4, 5] |_

# [1, 2, 3, 4, 5]

Here's a simplified version of the source:

class Bookend():
  def __or__(self, operand):
    return Pipe(operand)


class Pipe():
  def __init__(self, operand):
    self.operand = operand

  def __or__(self, f):
    if isinstance(f, Bookend):
      return self.operand
    else:
      self.operand = f(self.operand)
      return self


_ = Bookend()

Contact: @bzrry.