Re: Best use of "open" context manager

On 7/6/2024 6:49 AM, Rob Cliffe via Python-list wrote:

Consider this scenario (which I ran into in real life):
     I want to open a text file and do a lot of processing on the lines of that file.
     If the file does not exist I want to take appropriate action, e.g. print an error message and abort the program.
I might write it like this:
 try:
     with open(FileName) as f:
         for ln in f:
             print("I do a lot of processing here")
             # Many lines of code here .....
except FileNotFoundError:
     print(f"File {FileName} not found")
     sys.exit()
 but this violates the principle that a "try" suite should be kept small, so that only targeted exceptions are trapped,
not to mention that having "try" and "except" far apart decreases readability.
 Or I might write it like this:
 try:
     f = open(FileName) as f:
     FileLines = f.readlines()
except FileNotFoundError:
     print(f"File {FileName} not found")
     sys.exit()
# I forgot to put "f.close()" here -:)
for ln in File Lines:
         print("I do a lot of processing here")
         # Many lines of code here .....
 but this loses the benefits of using "open" as a context manager,
and would also be unacceptable if the file was too large to read into memory.
 Really I would like to write something like
 try:
     with open(FileName) as f:
except FileNotFoundError:
     print(f"File {FileName} not found")
     sys.exit()
else: # or "finally:"
         for ln in f:
             print("I do a lot of processing here")
             # Many lines of code here .....
 but this of course does not work because by the time we get to "for ln in f:" the file has been closed so we get
ValueError: I/O operation on closed file
 I could modify the last attempt to open the file twice, which would work, but seems like a kludge (subject to race condition, inefficient).
 Is there a better / more Pythonic solution?

I usually read the file into a sequence of lines and then leave the open() as soon as possible.  Something like this:
FILENAME = 'this_is_an_example.txt'
lines = None
if os.path.exists(FILENAME):
     with open(FILENAME) as f:
         lines = f.readlines()
# do something with lines
Of course, if you want to read a huge number of lines you will need to be more thoughtful about it.  Or make all the processing within the open() block be a function.  Then you just have one more line in the block.