cell-var-from-loop问题和解决方案

cell-var-from-loop

problem
When using a loop variable in a closure, the value of this variable will be the same every time(the last value the variable had before the closed function/lambda is called).

funcs = [lambda x: x + elem for elem in range(3)]
results = [func(1) for func in funcs]
# results == [3, 3, 3]

The problem has nothing to do with lambdas or comprehensions. You’ll get the same behavior this way:

funcs = []
for elem in range(3):
	def func(x):
		return x + elem
	funcs.append(func)
results = [func(1) for func in funcs]

In most languages, you can solve the problem by wrapping the function definition inside another function that you define and call(for a function parameter, the function get the value of the parameter not the reference of the variable):

def make_func(elem):
	return lambda x: x + elem
funcs = [make_func(elem) for elem in range(3)]
results = [func(1) for func in funcs]

In python, we can write as follow:

funcs = [lambda x, j=elem: x + j for elem in range(3)]
results = [func(1) for func in funcs]

That’s because the default parameter value j gets a reference to the value of elem in the defining scope, not a closure cell capturing the variable elem