On Mon, Sep 18, 2017 at 11:30 PM, Antoon Pardon <antoon.par...@vub.be> wrote: > Op 18-09-17 om 14:58 schreef Chris Angelico: >> On Mon, Sep 18, 2017 at 10:42 PM, Rick Johnson >> <rantingrickjohn...@gmail.com> wrote: >>> (2) Python is, in essence, converting the source code >>> syntax of: >>> >>> if someObject: >>> >>> to: >>> >>> if BOOLEAN_VALUE_OF_SOMEOBJECT: >>> >>> Which, although the result of such a conversion (a >>> Boolean) will be perfectly symmetrical with formal logic >>> statements, the conversion is not obvious, because it >>> happens behind the curtains. >> What a surprise. In the context of a statement that has exactly two >> possible courses of action (either you go into the 'if' block, or you >> go into the 'else' block (if any)), you use the boolean value of an >> object. What part of this is not obvious? > > Well that you reduce an object to a boolean value is not obvious to > begin with. A TypeError because you are treating a non-boolean as > a boolean would have been more obvious to me.
Sure, but those are basically your only two options. Either you have a way of interpreting some/all objects as booleans, or you require an explicit conversion. In Python syntax: # Interpret all objects as boolean: if x: pass # Require explicit conversion or comparison: if bool(x): pass if x == True: pass But not BOTH of the above: if bool(x) == True: pass That's pointless. It's not "more explicit". It's just more redundant. > A second thought is that it isn't obvious that empty strings, lists ... > should be thought of as falsy. Sometimes I am treating a stream of > values/objects and when I ask for the next available items, i get > a string/list. An empty string/list in that context would mean that > nothing was available, but it is possible that more will be available > later and so could be treated just like a non-empty list/string. Yep. That's a philosophical choice; Python has decided that a container with nothing in it is falsy, on the assumption that you're more likely to be contrasting [1,2,3] against [] than either of the above against None. Other languages have made other choices. Python is 100% consistent within its native data types - including that, unless otherwise specified, an object is truthy and the absence of an object (eg None) is falsy. One notable violation of that rule was that a datetime.time() representing midnight used to be considered false, but that was treated as a bug and corrected, since a time of midnight is not equivalent to a number of zero, but to a point on a clock. > Once you understand how python does things, you can work with it, but > IMO the way python does things is IMO not at all that obvious as people > would like us to think. Understood - but it's not arbitrary, and you *can* get to know the rules. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
