带枚举的基本python file-io变量

Question

New to python and trying to learn the ropes of file i/o.

I am working with pulling lines from a large (2 million line) file in this format:

56fr4
4543d
4343d
5irh3

Here is the function I'm using to return a code:

def getCode(i):
    with open("test.txt") as f:
        for index, line in enumerate(f):
            if index == i:
                code = # what does it equal?
                break
    return code

Once the index gets to the right spot (i), what syntax do I use to set the code variable?

Answer 1

code = line.strip()

will assign code to the line number that is equal to i while removing the trailing new line.

you also need to update your code a bit

 def getCode(i):
    with open('temp.txt', 'r') as f:
             for index, line in enumerate(f):
                     if index == i:
                             code = line.strip()
                             return code

>>> def getCode(i):
...     with open('temp.txt') as f:
...             for index, line in enumerate(f):
...                     if index == i:
...                             code = line
...                             return code
 ... 
>>> getCode(2)
"                  'LINGUISTIC AFFILIATION',\n"

yes, " 'LINGUISTIC AFFILIATION'," is in my current temp.txt'

Answer 2

enumerate transforms one iterator into another, such that the things you iterate over become pairs of (numeric ID, original item from the underlying iterator).

In our case:

for index, line in enumerate(f):

f is the file object. File objects are iterators: they iterate over lines of the file.

We transform that into an iterator over (line number, line from file) pairs.

The for loop syntax iterates over the iterator, and assigns the (line number, line from file) pair to the variables: (index, line). This is just the normal behaviour of assigning one tuple to another.

So, each time through the loop, index gets assigned a line number, and line gets assigned the corresponding line from the file. The line from the file is what you want (possibly with some formatting), and line contains it, so...

If any of the above didn't make sense, you probably need to re-read an introduction to the language.