Newcomer struggling with tutorial

[CPK Smithies]

Just joined this group, fascinated by Python. Many thanks to those who have
worked so hard on it!

One big problem, though: I was completely thrown by this, which I quote
from the Tutorial:

Comparisons can be chained. For example, a < b == c tests whether a is

less than b and moreover b equals c.

I threw a mental wobbly and wasted two hours over this:

a = -1
b = 77
c = 1

(a < b)
True

True == c
True

(a < b) == c
False

Unfortunately I was brought up with the belief that if A == B and B == C,
then A should == C.

I confess that after two hours worrying about this I have given up on
Python and uninstalled it. A shame: it looked so good!

CPKS

 

[Peter Hansen]

I threw a mental wobbly and wasted two hours over this:

a = -1
b = 77
c = 1

(a < b)
True

True == c
True

(a < b) == c
False

Unfortunately I was brought up with the belief that if A == B and B == C,
then A should == C.

Strangely enough it does. Your last line is incorrect. Try executing
it again, from scratch, without reassigning any of the three values
you originally entered, because that's surely what you have done to
get that result.

Using Python 2.3:

C:\>python
Python 2.3.1 (#47, Sep 23 2003, 23:47:32) [MSC v.1200 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.

True

I confess that after two hours worrying about this I have given up on
Python and uninstalled it. A shame: it looked so good!

A shame indeed, were it in fact how Python worked.

-Peter

 

[Dave Harrison]

a = -1

b = 77
c = 1

(a < b)
True

True == c
True

(a < b) == c
False

Unfortunately I was brought up with the belief that if A == B and B == C,
then A should == C.

I confess that after two hours worrying about this I have given up on
Python and uninstalled it. A shame: it looked so good!

well isnt that the defeatist way to go.

sounds like you're trying to compare two values that are not necessarily
the same. You're thinking in boolean logic, and trying to express it in
a totally different way.

You wanted :

a = 1
b = 2
c = 1
print ((a<2) & c)

So we are saying we want to have BOTH (a<2) AND (c) to be true.

Dave

 

[Andrew Bennetts]

Just joined this group, fascinated by Python. Many thanks to those who have
worked so hard on it!

One big problem, though: I was completely thrown by this, which I quote
from the Tutorial:

less than b and moreover b equals c.

I threw a mental wobbly and wasted two hours over this:

a = -1
b = 77
c = 1

(a < b)
True

True == c
True

(a < b) == c
False

Did you type this from memory, rather than copying and pasting? This is
what I see:
False


Do you see the distinction? "a < b == c" is the same as writing "(a < b)
and (b == c)" in python, but "(a < b) == c" isn't. Perhaps the tutorial's
wording is unclear.

I confess that after two hours worrying about this I have given up on
Python and uninstalled it. A shame: it looked so good!

Then why are you posting here?

-Andrew.

 

[David Lees]

Just joined this group, fascinated by Python. Many thanks to those who have
worked so hard on it!

One big problem, though: I was completely thrown by this, which I quote
from the Tutorial:



less than b and moreover b equals c.

I threw a mental wobbly and wasted two hours over this:

a = -1
b = 77
c = 1

(a < b)
True

True == c
True

(a < b) == c
False

Unfortunately I was brought up with the belief that if A == B and B == C,
then A should == C.

I confess that after two hours worrying about this I have given up on
Python and uninstalled it. A shame: it looked so good!

CPKS

I have not read the tutorial, but the section you quote is quite clear
about what chaining means. I did not know about this, but it is kind of
neat. For example:
False

Python actually conforms more to standard math notation for transitivity
than whatever language you were 'brought up' on, which results in
confusion when you are used to standard algebraic notation. I still
recall 39 years ago being quite puzzled when I saw the statement:
LET A = A+1

Good luck with whatever language you choose. Personally, I have found
Python to be a wonderful practical tool for the occasional programmer
like me. And this group is a nice community for the isolated user
because someone usually supplies answers quickly and non-judgementally
(for the most part).

david

 

[Terry Reedy]

from the Tutorial:

This means that a < b == c is equivalent to
(a < b) and (b == c) and not
(a < b) == c., which is something else.

Chained operators are *not* 'associative' -- you cannot insert parens
unless you so for each overlapping pair of args and insert 'and'
between each.

I threw a mental wobbly and wasted two hours over this:

a = -1
b = 77
c = 1

(a < b)
True

True == c
True

For a < b == c, the second test Python makes is b==c (False).
But this matches for your next test.

(a < b) == c
False

For 2.2.1 (which does not have True/False on Windows .1

What version/machine were you running? Please cut and paste actual
output. If you have really discovered a bug in current Python, we
would want to know and fix it.

Unfortunately I was brought up with the belief that if A == B and B == C,
then A should == C.

Why 'unfortunately'? For builtin objects, this is true in Python.

I confess that after two hours worrying about this I have given up on
Python and uninstalled it. A shame: it looked so good!

If you really did that, it was a real shame.

Terry J. Reedy

 

[Jegenye 2001 Bt]

Like others have pointed out, Python does that chained comparison right.

I confess that after two hours worrying about this I have given up on
Python and uninstalled it. A shame: it looked so good!

CPKS

That seems to be a flame bait to me.
Perhaps not a good one because people didn't bite. Or we are just too
Pythonic (and squeeze instead)

Cheers,
Pm