在使用 readlines 和 .split() 函数时，使用 for 循环进行迭代会更容易吗？

Question

I've created a while loop to iterate through, read the data.txt file, calculate the perimeter/area/vertices, and the root will print out the answers.

I'm stuck here:

from Polygon import Polygon
from Rectangle import Rectangle

in_file = open('data.txt', 'r')

line = in_file.readlines()

while line:
    type_, num_of_sides = line.split( ) ###ERROR 'list' object has no attribute 'split'

    sides = [map(float, in_file.readline().split()) for _ in range(int(num_of_sides))]
    print(line)

    if type_ == 'P':
        poly_object = Polygon(sides)

    elif type_ == 'R':
        poly_object = Rectangle(sides)

    print('The vertices are {}'.format(poly_object.vertices))
    print('The perimeter is {}'.format(poly_object.perimeter()))
    print('The area is {}'.format(poly_object.area()))
    print()

    line = in_file.readline()

in_file.close()

Should I create a for loop that loops through since readlines is a list of strings and I want the split to read through each line? Or is it just the way that I'm choosing to format, so that's why I'm getting an error?

Answer 1

The immediate issue is use of readlines , not readline in line = in_file.readlines() . This will populate line with a list of all lines from the file, rather than a single line. This will lead to two issues with your program where the wrong type of data will propagate through the loop:

1. Calling while line , in this instance, after reading a non-empty file will cause the loop to be executed, because a non-empty list evaluates to true.
2. You call split on line . On the first iteration of the loop, line contains a list of strings (where each string was a line in the file). Lists to do not have a split method – only the individual strings do. This call fails.
3. If the call to split had not failed and the loop had been allowed to run through once, the subsequent call to line = in_file.readline() would not return the next line in the file, as all lines have been read by the earlier call to readlines and the cursor not reset in the intervening period from EOF. The loop will terminate.

If you did not have the final call in step 3, the loop would instead run forever without termination, as the value of line would never be updated to be a non-empty list.

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A minimal change is to adjust the initial call which assigns a value to the line variable to readline() rather than readlines() to ensure only a single line is read from the file. The logic in the code should then work.

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You may find the following implementation logic easier to implement and reason about:

• collect the machinery for reading from in_file together into a single block before the loop, using a context manager to automatically close the file on leaving the block
• producing a list of lines, which a simple for loop is sufficient to iterate over

# This context manager will automatically ensure `in_file` is closed
# on leaving the `with` block.
with open('data.txt', 'r') as in_file:
    lines = in_file.readlines()

for line in lines:
    # Do something with each line; in particular, you can call
    # split() on line without issue here.
    #
    # You do not require any further calls to in_file.readline()
    # in this block.