Python: 创建无注释文件拷贝

Question

对Python还是个新手，试着学习一本书的例子。这将创建一个文本文件副本，从以#注释开头的所有行中删除。它是这样的（包括我的实习生评论）：

# this should be able to (create a file if not present yet and) take a file and then write another one with the same contents stripped off comments
# I wasnt able to bring it into a proper work - resulting file empty

f = open("test.dat","w")
# write several lines (the new-line n-symbol)
f.write("line one\nline two\nline three\n# blah blah \n# blah")
#
f.close()
# readline method reads all the characters up to and including the next newline character:
f = open("test.dat","r")
print ( f.read() )
print()
# readlines returns lines including newline character


newf = open("test2.dat","w")
newf.close()
newf = open("test2.dat","r")

while True:
  text = f.readline()
  if text == "":
    break
  if text == "#":
    continue
  newf.write(text)
f.close()
newf.close()

print()

newf = open("test2.dat","r")
print (newf.read())
newf.close()

但是生成的文件是空的并且有0b。我能谦虚地问一下有什么问题吗？谢谢！你知道吗

Answer 1

您的代码有几个问题：

• 您打开了输入文件进行读取，并在print(f.read())中使用了它的全部内容；文件指针现在位于文件的末尾。

• 输出文件被打开进行写入，但随后立即关闭，从而创建了一个空文件。然后打开此空文件以读取。

• 循环一开始就退出了，因为文件末尾的readline()将返回一个空字符串''

• 你的if不会检查每行的第一个字符-而是将整行与#匹配。由于行还包含换行符，因此即使行上的#也不会与此条件匹配（readline会返回'#\n'）

---

您的案例的惯用代码可以是

with open('test.dat', 'w') as output_file:
    # write several lines (the new-line n-symbol)
    output_file.write("line one\nline two\nline three\n# blah blah \n# blah")
# file closed automatically

with open('test.dat') as input_file:
    print(input_file.read())
    print()
# closed automatically

# reopen input file, open output file
with open('test.dat') as input_file, open('test2.dat', 'w') as output_file:
    for line in input_file:
        if not line.startswith('#'):
            output_file.write(line) 
# both files again closed automatically at the end of with block

print('Contents of test2.dat are now:')
with open('test2.dat') as input_file:
    print(input_file.read())