Python中没有多行Lambda：为什么不呢？

I’ve heard it said that multiline lambdas can’t be added in Python because they would clash syntactically with the other syntax constructs in Python. I was thinking about this on the bus today and realized I couldn’t think of a single Python construct that multiline lambdas clash with. Given that I know the language pretty well, this surprised me.

Now, I’m sure Guido had a reason for not including multiline lambdas in the language, but out of curiosity: what’s a situation where including a multiline lambda would be ambiguous? Is what I’ve heard true, or is there some other reason that Python doesn’t allow multiline lambdas?

Look at the following:

map(multilambda x:
      y=x+1
      return y
   , [1,2,3])

Is this a lambda returning (y, [1,2,3]) (thus map only gets one parameter, resulting in an error)? Or does it return y? Or is it a syntax error, because the comma on the new line is misplaced? How would Python know what you want?

Within the parens, indentation doesn’t matter to python, so you can’t unambiguously work with multilines.

This is just a simple one, there’s probably more examples.

Guido van Rossum (the inventor of Python) answers this exact question himself in an old blog post.
Basically, he admits that it’s theoretically possible, but that any proposed solution would be un-Pythonic:

“But the complexity of any proposed solution for this puzzle is immense, to me: it requires the parser (or more precisely, the lexer) to be able to switch back and forth between indent-sensitive and indent-insensitive modes, keeping a stack of previous modes and indentation level. Technically that can all be solved (there’s already a stack of indentation levels that could be generalized). But none of that takes away my gut feeling that it is all an elaborate Rube Goldberg contraption.”

This is generally very ugly (but sometimes the alternatives are even more ugly), so a workaround is to make a braces expression:

lambda: (
    doFoo('abc'),
    doBar(123),
    doBaz())

It won’t accept any assignments though, so you’ll have to prepare data beforehand. The place I found this useful is the PySide wrapper, where you sometimes have short callbacks. Writing additional member functions would be even more ugly. Normally you won’t need this.

Example:

pushButtonShowDialog.clicked.connect(
    lambda: (
    field1.clear(),
    spinBox1.setValue(0),
    diag.show())

A couple of relevant links:

For a while, I was following the development of Reia, which was initially going to have Python’s indentation based syntax with Ruby blocks too, all on top of Erlang. But, the designer wound up giving up on indentation sensitivity, and this post he wrote about that decision includes a discussion about problems he ran into with indentation + multi-line blocks, and an increased appreciation he gained for Guido’s design issues/decisions:

http://www.unlimitednovelty.com/2009/03/indentation-sensitivity-post-mortem.html

Also, here’s an interesting proposal for Ruby-style blocks in Python I ran across where Guido posts a response w/o actually shooting it down (not sure whether there has been any subsequent shoot down, though):

http://tav.espians.com/ruby-style-blocks-in-python.html

[Edit] Read this answer. It explains why multiline lambda is not a thing.

Simply put, it’s unpythonic. From Guido van Rossum’s blog post:

I find any solution unacceptable that embeds an indentation-based block in the middle of an expression. Since I find alternative syntax for statement grouping (e.g. braces or begin/end keywords) equally unacceptable, this pretty much makes a multi-line lambda an unsolvable puzzle.

Let me present to you a glorious but terrifying hack:

import types

def _obj():
  return lambda: None

def LET(bindings, body, env=None):
  '''Introduce local bindings.
  ex: LET(('a', 1,
           'b', 2),
          lambda o: [o.a, o.b])
  gives: [1, 2]

  Bindings down the chain can depend on
  the ones above them through a lambda.
  ex: LET(('a', 1,
           'b', lambda o: o.a + 1),
          lambda o: o.b)
  gives: 2
  '''
  if len(bindings) == 0:
    return body(env)

  env = env or _obj()
  k, v = bindings[:2]
  if isinstance(v, types.FunctionType):
    v = v(env)

  setattr(env, k, v)
  return LET(bindings[2:], body, env)

You can now use this LET form as such:

map(lambda x: LET(('y', x + 1,
                   'z', x - 1),
                  lambda o: o.y * o.z),
    [1, 2, 3])

which gives: [0, 3, 8]

I’m guilty of practicing this dirty hack in some of my projects which is bit simpler:

    lambda args...:( expr1, expr2, expr3, ...,
            exprN, returnExpr)[-1]

I hope you can find a way to stay pythonic but if you have to do it this less painful than using exec and manipulating globals.

Let me try to tackle @balpha parsing problem. I would use parentheses around the multiline lamda. If there is no parentheses, the lambda definition is greedy. So the lambda in

map(lambda x:
      y = x+1
      z = x-1
      y*z,
    [1,2,3]))

returns a function that returns (y*z, [1,2,3])

But

map((lambda x:
      y = x+1
      z = x-1
      y*z)
    ,[1,2,3]))

means

map(func, [1,2,3])

where func is the multiline lambda that return y*z. Does that work?

(For anyone still interested in the topic.)

Consider this (includes even usage of statements’ return values in further statements within the “multiline” lambda, although it’s ugly to the point of vomiting ;-)

>>> def foo(arg):
...     result = arg * 2;
...     print "foo(" + str(arg) + ") called: " + str(result);
...     return result;
...
>>> f = lambda a, b, state=[]: [
...     state.append(foo(a)),
...     state.append(foo(b)),
...     state.append(foo(state[0] + state[1])),
...     state[-1]
... ][-1];
>>> f(1, 2);
foo(1) called: 2
foo(2) called: 4
foo(6) called: 12
12

You can simply use slash (\) if you have multiple lines for your lambda function

Example:

mx = lambda x, y: x if x > y \
     else y
print(mx(30, 20))

Output: 30

I am starting with python but coming from Javascript the most obvious way is extract the expression as a function….

Contrived example, multiply expression (x*2) is extracted as function and therefore I can use multiline:

def multiply(x):
  print('I am other line')
  return x*2

r = map(lambda x : multiply(x), [1, 2, 3, 4])
print(list(r))

https://repl.it/@datracka/python-lambda-function

Maybe it does not answer exactly the question if that was how to do multiline in the lambda expression itself, but in case somebody gets this thread looking how to debug the expression (like me) I think it will help

On the subject of ugly hacks, you can always use a combination of exec and a regular function to define a multiline function like this:

f = exec('''
def mlambda(x, y):
    d = y - x
    return d * d
''', globals()) or mlambda

You can wrap this into a function like:

def mlambda(signature, *lines):
    exec_vars = {}
    exec('def mlambda' + signature + ':\n' + '\n'.join('\t' + line for line in lines), exec_vars)
    return exec_vars['mlambda']

f = mlambda('(x, y)',
            'd = y - x',
            'return d * d')

I was just playing a bit to try to make a dict comprehension with reduce, and come up with this one liner hack:

In [1]: from functools import reduce
In [2]: reduce(lambda d, i: (i[0] < 7 and d.__setitem__(*i[::-1]), d)[-1], [{}, *{1:2, 3:4, 5:6, 7:8}.items()])                                                                                                                                                                 
Out[3]: {2: 1, 4: 3, 6: 5}

I was just trying to do the same as what was done in this Javascript dict comprehension: https://stackoverflow.com/a/11068265

Here’s a more interesting implementation of multi line lambdas. It’s not possible to achieve because of how python use indents as a way to structure code.

But luckily for us, indent formatting can be disabled using arrays and parenthesis.

As some already pointed out, you can write your code as such:

lambda args: (expr1, expr2,... exprN)

In theory if you’re guaranteed to have evaluation from left to right it would work but you still lose values being passed from one expression to an other.

One way to achieve that which is a bit more verbose is to have

lambda args: [lambda1, lambda2, ..., lambdaN]

Where each lambda receives arguments from the previous one.

def let(*funcs):
    def wrap(args):
        result = args                                                                                                                                                                                                                         
        for func in funcs:
            if not isinstance(result, tuple):
                result = (result,)
            result = func(*result)
        return result
    return wrap

This method let you write something that is a bit lisp/scheme like.

So you can write things like this:

let(lambda x, y: x+y)((1, 2))

A more complex method could be use to compute the hypotenuse

lst = [(1,2), (2,3)]
result = map(let(
  lambda x, y: (x**2, y**2),
  lambda x, y: (x + y) ** (1/2)
), lst)

This will return a list of scalar numbers so it can be used to reduce multiple values to one.

Having that many lambda is certainly not going to be very efficient but if you’re constrained it can be a good way to get something done quickly then rewrite it as an actual function later.

because a lambda function is supposed to be one-lined, as its the simplest form of a function, an entrance, then return