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?
Best wishes
Rob Cliffe
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