What is the most pythonic way to open a file?

Question

I'm trying to clean up my code a little bit, and I have trouble figuring which of these 2 ways is considered the most pythonic one

import os

dir = os.path.dirname(__file__)
str1 = 'filename.txt'
f = open(os.path.join(dir,str1),'r')

Although the second seems to be cleanest one, I find the declaration of fullPath a bit too much, since it will only be used once.

import os

dir = os.path.dirname(__file__)
str1 = 'filename.txt'
fullPath = os.path.join(dir,str1)
f = open(fullPath,'r')

In general, is it a better thing to avoid calling functions inside of another call, even if it adds a line of code ?

Answer 1

with open('file path', 'a') as f:
   data = f.read()
   #do something with data

or

f = open(os.path.join(dir,str1),'r')
f.close()

Answer 2

file = open('newfile.txt', 'r') 

for line in file:

      print line

OR

lines = [line for line in open('filename')]

If file is huge, read() is definitively bad idea, as it loads (without size parameter), whole file into memory.

If your file is huge this will cause latency !

So, i don't recommend read() or readlines()