为什么''显然在Python中评估为True

Question

I have:

raw_matrix.append([])
for as_string in ' '.split(line):
    if as_string:
        print('as_string: #' + as_string + '#')
        raw_matrix[-1].append(int(as_string))

This produces the output:

as_string: # #
Traceback (most recent call last):
  File "product_of_four", line 27, in <module>
    raw_matrix[-1].append(int(as_string))
ValueError: invalid literal for int() with base 10: ''

raw_matrix is a 20x20 array of lines of two digit (decimal) numbers separated by spaces.

If I'm reading this correctly, as_string is evaluating to ' ' or '', and I'm getting an exception as a side effect of as_string not being parseable as the side effect of it not containing an int()-parseable digit string.

How can I change things so that Python 2.x (3.x) parses a string of two-digit integers, rather than trying to parse unparseable strings as integers?

Thanks,

Answer 1

The line:

for as_string in ' '.split(line):

is pretty fishy here. You're splitting the string ' ' on the delimiter line , most likely returning the list [' '] . Remember that strings only evaluate to False-y values when they are empty (0 characters). The string ' ' is not empty (it has one character).

You probably wanted to do something like:

for as_string in line.split():
    ...

which will split line on runs of consecutive whitespace.

Answer 2

You are using split the wrong way around; split the line, not the ' ' space:

for as_string in line.split(' '):

By splitting the space character by line delimiters, you invariably are going to end up with [' '] , not an empty string:

>>> ' '.split('10 20 30')
[' ']

(the exceptions being an empty delimiter, not allowed, and splitting on ' ' , which gives you two empty strings instead). And the one and only element in the result, ' ' , is a non-empty string, so True in a boolean context.

As for the error message, int() ignores leading and trailing whitespace. What is left is an empty string:

>>> int(' ')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: ''

You probably want to even omit the ' ' argument wen splitting and just split on variable-width whitespace:

for as_string in line.split():

leaving the first argument set to the default, None . Using str.split() also ignores any leading and trailing whitespace, always handy when reading lines from a file, those would include a newline:

>>> '  10          20\t30\n'.split()
['10', '20', '30']

From the documentation:

If sep is not specified or is None , a different splitting algorithm is applied: runs of consecutive whitespace are regarded as a single separator, and the result will contain no empty strings at the start or end if the string has leading or trailing whitespace. Consequently, splitting an empty string or a string consisting of just whitespace with a None separator returns [] .

Answer 3

Your split statement is constructed incorrectly; it should be line.split(' ') . The way it's written, you're trying to divide up a single space by whatever the contents of line are, which is obviously just going to return the space. That can't be evaluated as an integer.

Answer 4

' ' evaluates to True because it is not an empty string (there is a space in there).

The problem with your code is that for as_string in ' '.split(line) should in fact be for as_string in ' '.split(line)