Python: Define a variable inside a "lambda"

Question

So, I tried to search here and couldn't find any solution to that problem: I need to define a variable inside a "lambda".

I have that piece of code:

def z(t,s):exec("t=s")
r = type('', (), {
    '__init__': lambda t,*s: z("t.c",s),
    's': lambda t: (lambda t,*s: [t[::-1] for t in s])(*t.c),
    'l': lambda t: (lambda t,*s: list(s[::-1]))(*t.c),
    'd': lambda t: t.c
})

w = ["APPLE", "BEACH", "CITRUS"]
print r(*w).s()
print r(*w).l()
print r(*w).d()

Nothing works as it should work... Also when I call the function "d", it returns me this error:

AttributeError: '' object has no attribute 'c'

When it should return a tuple, something like:

('ALPHA', 'BRAVO', 'CHARLIE')

EDIT: Thanks guys, it works now, and is even smaller:

r=type('',(),{'__init__':lambda t,*s:setattr(t,'c',s),'s':lambda t:[t[::-1] for t in t.c],'l':lambda t:list(t.c[::-1]),'d':lambda t:t.c})

Why the heck are you doing this instead of just using a class statement? And are you aware that def can do literally anything lambda can? — user2357112
– user2357112, Commented Sep 14, 2014 at 4:17
Because it's a one-liner: def z(t,s):exec("t=s");r=type('',(),{'c':'','__init__':lambda t,*s:z(t.c,s),'s':lambda t:(lambda t,*s:[t[::-1] for t in s])(*t.c),'l':lambda t:(lambda t,*s:list(s[::-1]))(*t.c),'d':lambda t:t.c}) — William Fernandes
– William Fernandes, Commented Sep 14, 2014 at 4:18
"Because it's a one-liner" isn't a good reason to do things, especially when your one-liner doesn't fit on a line anyway. — user2357112
– user2357112, Commented Sep 14, 2014 at 4:20

falsetru · Accepted Answer · 2014-09-14 04:17:57Z

2

You need to use setattr to set an attribute: (If you want something like self.c = s)

'__init__': lambda t,*s: setattr(t, 'c', s),

>>> r = type('', (), {
...     '__init__': lambda t,*s: setattr(t, 'c', s),
...     's': lambda t: (lambda t,*s: [t[::-1] for t in s])(*t.c),
...     'l': lambda t: (lambda t,*s: list(s[::-1]))(*t.c),
...     'd': lambda t: t.c
... })
>>>
>>> w = ["APPLE", "BEACH", "CITRUS"]
>>> print r(*w).s()
['HCAEB', 'SURTIC']
>>> print r(*w).l()
['CITRUS', 'BEACH']
>>> print r(*w).d()
('APPLE', 'BEACH', 'CITRUS')

answered Sep 14, 2014 at 4:17

falsetru

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3 Comments

falsetru Over a year ago

@WilliamFernandes, Inner lambdas in s and l is redundant: 's': lambda t: [t[::-1] for t in t.c[1:]] , 'l': lambda t: list(t.c[1:][::-1])

falsetru Over a year ago

@WilliamFernandes, Maybe you wanted this? 's': lambda t: [t[::-1] for t in t.c], 'l': lambda t: list(t.c[::-1]),

falsetru Over a year ago

@WilliamFernandes, As others commented, avoid one-liner. It's hard to understand; hard to maintain.

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