Python似乎自动将列表转换为字符串，之后.append（）和.pop（）不可用

Question

I'm using Python 3.6 and PyCharm if that matters at all. I'm also fairly new to Python programming (especially working with files) so sorry if I'm missing something obvious.

I am trying to make a script that writes the first 20 items from the look-and-say sequence into a file, each item on a separate line. The script works fine for the first 4 lines, however, raises an exception afterwards.

LINES = 20
with open("sequence.txt", "r+") as finalOutput:
    finalOutput.truncate(0)
    finalOutput.write("1")
    for i in range(LINES-1):
        finalOutput.seek(0)
        prevLine = finalOutput.readlines()[-1]
        newLine = ""
        block = []
        for c in prevLine:
            if c == "\n":
                break
            else:
                block.append(c)
                if block[-1] != block[0]:
                    newLine += str(len(block[:-1]))
                    newLine += str(block[0])
                    block = block.pop()
        if len(block) != 0:
            newLine += str(len(block))
            newLine += str(block[0])
        finalOutput.write("\n" + newLine)

After "1211" is written into the file, I'm presented with the following error:

Traceback (most recent call last):
  File "C:/Users/ViAik/PycharmProjects/1_11_21_1211_sequence/main.py", line 
14, in <module>
    block.append(c)
AttributeError: 'str' object has no attribute 'append'

Which implies that block is a string, even though it started out as a list (?). What confuses me the most is that the first 4 lines work properly.

I am not even sure how to formulate my actual question here because I don't understand what's actually going on. I suppose that figuring that out is my question, and hopefully I can fix this code in a way that doesn't involve purposefully using block as a string. Any help? Thanks in advance.

Answer 1

block = block.pop()

This makes block become it's own last element, which is a string. I'm not sure what you expect to happen here.

eg

block = ['hello', 'world']
block = block.pop()
print(block)  # --> world

Answer 2

Python works on the concept of reference object. Variable names are references to the actual object. When a name is on the right hand side of an equation, the object that it references is automatically looked up and used in the equation. So when you write:

 block = [] # the name block is a reference to a list

But when you again reference name block to a new object, it will point to the last reference object it assign.

block = block.pop() # the name block is a reference to string object.

So when you try to append an element in block it gives you error.