使用索引，打开（文件）作为枚举（文件指针）

Question

I'm trying to open a file using the with syntax while also enumerating the file's lines.

So I tried this:

with index, open(filename) as enumerate(f):
    f.read()...

Where f is meant to correspond to open(filename) , and index should take enumerate(f) . So I want to be able to call f.read() in the body, and know which line of the file is being processed.

I'm sure this can be done -- I'm just not using the syntax correctly. Any help?

Answer 1

I think the closest thing that's valid Python to what you're trying is this:

with open(filename) as f:
    for index, line in enumerate(f):
        # Do stuff with each line.

f.read() reads the entire contents of f and stores it in a string, so you wouldn't be able to get line numbers that way.

You can't combine the enumerate call with the with statement because the language's grammar just doesn't allow it :

 with_stmt ::= "with" with_item ("," with_item)* ":" suite with_item ::= expression ["as" target] 

The execution of the with statement with one “item” proceeds as follows:

1. The context expression (the expression given in the with_item) is evaluated to obtain a context manager.

2. The context manager's __exit__() is loaded for later use.

3. The context manager's __enter__() method is invoked.

4. If a target was included in the with statement, the return value from __enter__() is assigned to it.

target is meant to be a variable that the return value of open(filename) is assigned to. It can't be a function call.

Answer 2

You seem to be confusing the with statement with the for statement. In

for i, x in enumerate(lst):

i and x are set to the values obtained from the iterator created by enumerate . In

with open(filename) as f:

f is assigned the return value of open .

You cannot mix the two. You need to use two separate statements:

with open(filename) as f:
    for index, line in enumerate(f):