Opening a file with open(ARGV.first) vs declaring a variable

I am working through Exercise 20 of Learn Ruby the Hard Way, and am trying to make sense of the difference in output between defining the variable "current_file" and simply using open(ARGV.first):

Here is the input file test.txt:

This is line 1
This is line 2
This is line 3

Here is the beginning of the script:

input_file = ARGV.first

def print_a_line(f)
  puts "#{f.gets}"
end

current_file = open(input_file)

When I call the print_a_line with the declared variable:

print_a_line(current_file)
print_a_line(current_file)
print_a_line(current_file)

Then f.gets tracks to the new line each time:

This is line 1
This in line 2
This is line 3

However, if I call the function with ARGV.first:

print_a_line(open(ARGV.first))
print_a_line(open(ARGV.first))
print_a_line(open(ARGV.first))

Then f.gets does not track to the new line and remains on line 1:

This is line 1
This is line 1
This is line 1

And the same even with input_file:

print_a_line(open(input_file))
print_a_line(open(input_file))
print_a_line(open(input_file))

f.get still doesn't track to the next line:

This is line 1
This is line 1
This is line 1

Obviously, I would declare and use variables -- I understand the basics of writing neat, readable code.

My question is why does f.get track to the next line when the function is called with a declared variable and not track to the next line when called with the open(ARGV.first) or open(input_file)?

Why do I have to declare the variable again?

current_file = open(input_file)

ruby code study

Published on 07 Dec 2016 by Alexander Garber

Opening a file with open(ARGV.first) vs declaring a variable

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