On 13/05/2022 18:37, bryangan41 wrote:
Is the following LBYL:foo = 123if foo < 200: do()If so, how to change to
EAFP?Thanks!Sent from Samsung tablet.
The distinction between look-before-you-leap and
easier-to-ask-forgiveness-than-permission is weaker than yo might expect.
When you write
filename = ...
if exists(filename):
with open(filename) as instream:
# do stuff
else:
# fallback
there are two checks for the file's existence, one explicit, and one
implicitly inside open() -- and worse, the first, explicit, check is
unreliable because between exists() and open() there is a small delay
that may be sufficient to create or delete the file. Therefore the
recommended (EAFP) version of the above is
filename = ...
try:
with open(filename) as instrem:
# do stuff
except FileNotFoundError
# fallback
or just
with open(filename) as instream:
# do stuff
if there is no meaningful fallback.
Regarding your example code, whether you can write an EAFP version for
if foo < 200:
do()
depends on the contents of do(). If do() fails in a well-defined way,
let's say by raising a FooAbove199 exception you can write just
do()
or, if you need a fallback
try:
do()
except FooAbove199:
# fallback
Note that if you change do() from
do():
# do stuff
to
def do():
if foo < 200:
# do stuff
else:
raise FooAbove199(f"Invalid {foo=!r}")
the
do()
invocation becomes EAFP even though you are actually performing the same
test as before.
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