Passing 'None' as function parameter (where parameter is a function)
I am writing a small app that has to perform some 'sanity checks' before entering execution. (eg. of a sanity check: test if a certain path is readable / writable / exists)
The code:
import logging
import os
import shutil
import sys
from paths import PATH
logging.basicConfig(level=logging.DEBUG)
log = logging.getLogger('sf.core.sanity')
def sanity_access(path, mode):
ret = os.access(path, mode)
logfunc = log.d开发者_JAVA百科ebug if ret else log.warning
loginfo = (os.access.__name__, path, mode, ret)
logfunc('%s(\'%s\', %s)==%s' % loginfo)
return ret
def sanity_check(bool_func, true_func, false_func):
ret = bool_func()
(logfunc, execfunc) = (log.debug, true_func) if ret else \
(log.warning, false_func)
logfunc('exec: %s', execfunc.__name__)
execfunc()
def sanity_checks():
sanity_check(lambda: sanity_access(PATH['userhome'], os.F_OK), \
lambda: None, sys.exit)
My question is related to the sanity_check
function.
This function takes 3 parameters (bool_func
, true_func
, false_func
). If the bool_func
(which is the test function, returning a boolean value) fails, true_func
gets executed, else the false_func
gets executed.
1) lambda: None
is a little lame , because for example if the sanity_access returns True, lambda: None
gets executed, and the output printed will be:
DEBUG:sf.core.sanity:access('/home/nomemory', 0)==True
DEBUG:sf.core.sanity:exec: <lambda>
So it won't be very clear in the logs what function got executed. The log will only contain <lambda>
. Is there a default function that does nothing and can be passed as a parameter ? Is it a way to return the name of the first function that is being executed inside a lambda ?
Or a way not to log that "exec" if 'nothing' is sent as a paramter ?
What's the none / do-nothing equivalent for functions ?
sanity_check(lambda: sanity_access(PATH['userhome'], os.F_OK), \
<do nothing, but show something more useful than <lambda>>, sys.exit)
Additional question, why is lambda: pass
instead of lambda: None
not working ?
What's with all the lambdas that serve no purpose? Well, maybe optional arguments will help you a bit:
def sanity_check( test, name='undefined', ontrue=None, onfalse=None ):
if test:
log.debug(name)
if ontrue is not None:
ontrue()
else:
log.warn( name )
if onfalse is not None:
onfalse()
def sanity_checks():
sanity_check(sanity_access(PATH['userhome'], os.F_OK), 'test home',
onfalse=sys.exit)
But you are really overcomplicating things.
update
I would normally delete this post because THC4k saw through all the complexity and rewrote your function correctly. However in a different context, the K combinator trick might come in handy, so I'll leave it up.
There is no builtin that does what you want AFIK. I believe that you want the K combinator (the link came up on another question) which can be encoded as
def K_combinator(x, name):
def f():
return x
f.__name__ = name
return f
none_function = K_combinator(None, 'none_function')
print none_function()
of course if this is just a one off then you could just do
def none_function():
return None
But then you don't get to say "K combinator". Another advantage of the 'K_combinator' approach is that you can pass it to functions, for example,
foo(call_back1, K_combinator(None, 'name_for_logging'))
as for your second statement, only expressions are allowed in lambda
. pass
is a statement. Hence, lambda: pass
fails.
You can slightly simplify your call to sanity check by removing the lambda around the first argument.
def sanity_check(b, true_func, false_func):
if b:
logfunc = log.debug
execfunc = true_func
else:
logfunc = log.warning
execfunc = false_func
logfunc('exec: %s', execfunc.__name__)
execfunc()
def sanity_checks():
sanity_check(sanity_access(PATH['userhome'], os.F_OK),
K_combinator(None, 'none_func'), sys.exit)
This is more readable (largely from expanding the ternary operator into an if). the boolfunc
wasn't doing anything because sanity_check
wasn't adding any arguments to the call. Might as well just call instead of wrapping it in a lambda.
You might want to rethink this.
class SanityCheck( object ):
def __call__( self ):
if self.check():
logger.debug(...)
self.ok()
else:
logger.warning(...)
self.not_ok()
def check( self ):
return True
def ok( self ):
pass
def not_ok( self ):
sys.exit(1)
class PathSanityCheck(SanityCheck):
path = "/path/to/resource"
def check( self ):
return os.access( path, os.F_OK )
class AnotherPathSanityCheck(SanityCheck):
path = "/another/path"
def startup():
checks = ( PathSanityCheck(), AnotherPathSanityCheck() )
for c in checks:
c()
Callable objects can simplify your life.
>>> import dis
>>> f = lambda: None
>>> dis.dis(f)
1 0 LOAD_CONST 0 (None)
3 RETURN_VALUE
>>> g = lambda: Pass
>>>
>>>
>>> dis.dis(g)
1 0 LOAD_GLOBAL 0 (Pass)
3 RETURN_VALUE
>>> g = lambda: pass
File "<stdin>", line 1
g = lambda: pass
^
SyntaxError: invalid syntax
Actually, what you want is a function which does nothing, but has a __name__
which is useful to the log. The lambda function is doing exactly what you want, but execfunc.__name__
is giving "<lambda>"
. Try one of these:
def nothing_func():
return
def ThisAppearsInTheLog():
return
You can also put your own attributes on functions:
def log_nothing():
return
log_nothing.log_info = "nothing interesting"
Then change execfunc.__name__
to getattr(execfunc,'log_info', '')
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