在python中，如何拆分元组并作为map函数的单独参数传递

Question

Here's what I want to do

class Vertex:
  def __init__(self, x,y):
    # ...

coords = [(0,0),(10,10)]
v0, v1 = map(lambda x,y: Vertex(x,y), coords)

The above code won't work, because the coord tuples get passed as x , and there is nothing for y

A workaround that works is

class Vertex:
  def __init__(self, x,y):
    # ...

coords = [(0,0),(10,10)]
v0, v1 = map(lambda coord: Vertex(coord[0],coord[1]), coords)

But the first solution looks more elegant to me. Probably I've to add an extra step to split the coord tuple. How do I do that?

Answer 1

You can use tuple-unpacking to call the Vertex constructor:

coord = (0, 0)
Vertex(*coord)

Or for your example:

v0, v1 = map(lambda coord: Vertex(*coord), coords)

Alternatively, since you are on Python 2, you can also make the lambda accept a tuple:

v0, v1 = map(lambda (x, y): Vertex(x, y), coords)

Instead of calling map (which actually returns a generator in Python 3, so your code wouldn't work there), you can also use a list comprehension to create the Vertex objects:

v0, v1 = [Vertex(*coord) for coord in coords]

# or unpack the coordinates as suggested by NPE
v0, v1 = [Vertex(x, y) for x, y in coords]

Answer 2

Just put parentheses around (x, y) :

v0, v1 = map(lambda (x,y): Vertex(x,y), coords)
#                   ^   ^

Edit: it turns out the above won't work in Python 3 (PEP 3113).

The following generator expression will work in both Python 2 and 3:

v0, v1 = (Vertex(x, y) for x, y in coords)

Answer 3

use zip along with unpack operator *:

map(Vertex, *zip(*coords))

also, your __init__ signature is possibly wrong, as I assume it misses self argument:

>>> class Vertex:
...     def __init__(self, x, y):
...         self.x, self.y = x, y
...     def __repr__(self):
...         return 'vertex({}, {})'.format(self.x, self.y)
... 
>>> 
>>> coords = [(1, 2), (3, 4)]
>>> tuple(map(Vertex, *zip(*coords)))
(vertex(1, 2), vertex(3, 4))