如何使其成为列表理解？

Question

I have a list of strings. Some of these strings can be converted to floats, other not. I'm trying to extract those that can be converted to floats. For now, I am using the following code

my_strings = ['1', '42', '.3', 'a', 'b', 'c']
my_floats = []
for e in my_strings:
    try:
        my_floats.append(float(e))
    except ValueError:
        pass

Is there a way of doing this operation as a list comprehension?

Answer 1

You could write a safeFloat function that converts a string to a float and uses a default value for invalid strings. This could be used in a list comprehension or mapped with a filter if you don't want items that aren't numeric:

def safeFloat(S,default=None):
    try:    return float(S)
    except: return default


my_strings = ['1', '42', '.3', 'a', 'b', 'c']

my_floats  = [safeFloat(s,0) for s in my_strings]

[1.0, 42.0, 0.3, 0, 0, 0]

my_floats = [float(s) for s in my_strings if safeFloat(s) != None]

[1.0, 42.0, 0.3]

my_floats  = list(filter(None,map(safeFloat,my_strings)))

[1.0, 42.0, 0.3]

If you're doing this often on lists or iterators, you can further encapsulate the conversion in a function that converts and filters multiple items

def getFloats(S,default=None):
    yield from filter(None,map(safeFloat,S))

my_floats = [*getFloats(my_strings)]

[1.0, 42.0, 0.3]

Answer 2

You should not rely on exceptions for your code logic. Try this:

my_strings = ['1', '42', '.3', 'a', 'b', 'c']
my_floats = [float(string) for string in my_strings if string.replace('.','',1).isdigit()]