在Python中，如何从文件创建2D列表并为每个子列表中的每个项目分配类型？

Question

Sorry if that's an awkward title. Here's what I want to do:

1. Read a txt file that on each line contains: country name, area, population
2. Each line turns into a sublist within a main list
3. Each sublist needs to retain the type of variable, so: string,float,int for each sublist

A sample of what would be a long output is:

[['Afghanistan', '647500.0', '25500100'], ['Albania', '28748.0', '2821977'], ['Algeria', '2381740.0', '38700000'], ['American Samoa', '199.0', '55519'], ['Andorra', '468.0', '76246']

etc... and python needs to know the type of each variable, for example if i wanted to create a new function that sorts all floats in the list.

Here's what I have now, but it only returns each line as a sublist, and I believe doesn't know the type if I wanted to use this 2d list for new functions.

def Countries(filename):
f = open(filename)
lines = f.readlines()
f.close()
myList = []
for line in lines:
    myList.append(line.strip().split(','))
return myList

Answer 1

Perhaps this is what you're looking for:

def Countries(filename):
    f = open(filename)
    myList = []
    for line in f.readlines():
        line_data = line.split(",")
        myList.append(line_data)
    return myList

But, if it's a .txt file I think it's only going to be returning strings. So, you'd have to convert types.

Answer 2

the main problem you have is that f.readlines() reads everything as a string. you can then map each item in your sublist through a type checker like this:

def typeCheck(s):
    try:
        return int(s)
    except ValueError:
        try:
            return float(s)
        except:
            return s

will try to explicitly cast your parameter as an 'int', then a 'float', then just leave it as is. it is this order because all things that can be cast as a 'int' can also be cast as a 'float' but not the other way around so you want to check for the int first (ie, '123.1' will raise an error for int('123.1') but not float('123.1') )

you can then append the mapped list like this:

for line in lines:
    myList.append(list(map(typeCheck,line.strip().split(',')))

and your values that can be converted to floats and int will be converted.

EXAMPLE:

>>> def typeCheck(s):
    try:
        return int(s)
    except ValueError:
        try:
            return float(s)
        except:
            return s


>>> l = [['Afghanistan', '647500.0', '25500100'], ['Albania', '28748.0', '2821977'], ['Algeria', '2381740.0', '38700000'], ['American Samoa', '199.0', '55519'], ['Andorra', '468.0', '76246']]
>>> for i in l: print(list(map(typeCheck,i)))

['Afghanistan', 647500.0, 25500100]
['Albania', 28748.0, 2821977]
['Algeria', 2381740.0, 38700000]
['American Samoa', 199.0, 55519]
['Andorra', 468.0, 76246]