Python read() 不读取文件

Question

So I made a simple script in Python which takes a filename as a command line argument and reads the given file whenever the script is executed. Pretty straight forward, whenever I execute the script it does not print the text inside the file, nor do I get an error. I am pretty new to programming and have been breaking my head for hours in this piece of code.

import sys

def openfile(filename):
    f = open(filename, mode='r')
    f.read()
    f.close()

if __name__ == '__main__':
        openfile(filename = sys.argv[1])
        print('script has been executed')

Quick disclaimer I know someone posted a similar question a few years ago, but he did not get a direct answer on why his code wasn't working and I did not get the information I was looking out for the answers. Also sorry for my poor English. I did my best to keep it clear, but English is not my first language.

Answer 1

You are reading the file f.read() but you need to assign the content to a variable or print it or something. Python won't print the content of every file you read just because you read it: you have to tell it what to do with the result.

Answer 2

For python newbie, studying the Python anti-patterns is a must. This allow one avoid many mistake and bugs. OP apparently has pass experience from other programming language but doesn't aware of better way of working in python.

If you study the anti-pattern, you will learn with open() is the better way to open a file. Immediately, you will spot f as a file handler, which will not store any data unless you pass data from f.read() function to another variable.

def openfile(filename):
    with open(filename) as f:
        data = f.read()
    # to print out the contents
    print(data)

Using sys.argv is acceptable, but argparse is highly recommended.

Answer 3

It appears as though the program simply reads the file that is passed into the command line argument. The program functions and reads the file, but you need a line to tell python to print the contents of the file you are reading. Try this:

import sys

def openfile(filename):
    f = open(filename, mode='r')
    contents = f.read()
    print(contents)
    f.close()

if __name__ == '__main__':
    openfile(filename=sys.argv[1])
    print('script has been executed')

Answer 4

You are reading the file but you have to assign the contents to some variable or you can just print the contents.

For Example:

f = open("SampleTextFile_100kb.txt", "rt") here r for read and t for text file

print(f.read()) # TO READ AND PRINT COMPLETE FILE.

print(f.read(55)) # TO READ UPTO SPECIFIED CHARACTERS LIMIT.

TO READ LINE/S OR THE WHOLE FILE AS LINES:

print(f.readline()) #TO READ MULTIPLE LINES print(f.readline()) print(f.readline())

TO READ THE WHOLE FILE AS LINES

for x in f:
print(x)

hope this helps.

Answer 5

I strongly recommend @mootmoot 's approach with the context manager. This is not only easier to read, but it also catches exceptions when the file cannot be read for some reason. If you don't want to use a context manager, I would suggest you use the try-except commands to for your function. It is unsafe if you do it like @Adam Merki suggested because there will be no guarantee that the file will be closed if an exception occurs while reading the file.

Using try-except statements, your function could look like this:

import sys

def openfile(filename):

    f = open(filename, mode='r')
    try:
        print(f.read())

    except:
        print('something went wrong')

    else:
        print('nothing went wrong')

    finally:
        f.close()


if __name__ == '__main__':
    openfile(filename=sys.argv[1])
    print('script has been executed')