the PHP ternary operator equivalent on Python

[Daniel Crespo]

Hi!

I would like to know how can I do the PHP ternary operator/statement
(... ? ... : ...) in Python...

I want to something like:

a = {'Huge': (quantity>90) ? True : False}

Any suggestions?

Thanks

Daniel

[Steven D'Aprano]

On Fri, 18 Nov 2005 21:05:50 -0800, bo****@gmail.co m wrote:

Steven D'Aprano wrote:

WHY WHY WHY the obsession with one-liners? What is wrong with the good old
fashioned way?

if cond:
x = true_value
else:
x = false_value

It is easy to read, easy to understand, only one of true_value and
false_value is evaluated. It isn't a one-liner. Big deal. Anyone would
think that newlines cost money or that ever time you used one God killed a
kitten.

How do you put this block into list comprehension or generator
expression ? Why the obsession of these block style ?

Why do you assume that everything you need for your list comprehension has
to go into a single line? Chances are your list comp already calls
functions, so just create one more for it to use.
py> def describe(cond):
.... if cond:
.... return "odd"
.... else:
.... return "even"
....
py> L = [describe(n % 2) for n in range(8)]
py> L
['even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd']

One major advantage is that this makes it easier to test your function
describe() in isolation, always a good thing.

Another advantage is that the idiom "call a function" is extensible to
more complex problems:

def describe(n):
if n < 0:
return "negative " + describe(-n)
elif n == 0:
return "zero"
elif n % 2:
return "odd"
else:
return "even"

L = [describe(n) for n in range(8)]

if much easier to understand and follow than using ternary expressions:

# obviously untested
L = ["zero" if n == 0 else \
"negative " + ("odd" if n % 2 else "even") if n < 0 else \
"odd" if n % 2 else "even" for n in range(8)]

Yes, I've seen ternary expressions nested three and even four deep.

I find it fascinating to read Guido's reasoning for introducing a ternary
statement. From the PEP here http://www.python.org/peps/pep-0308.html he
links to this comment of his:

[quote]
I think Raymond's example is more properly considered an argument for
adding a conditional expression than for removing the current behavior of
the and/or shortcut operators; had we had a conditional expression, he
wouldn't have tried to use the "x and y or z" syntax that bit him.
[end quote]

Looking back to Raymond's example here:
http://mail.python.org/pipermail/pyt...er/056510.html

[quote]
I propose that in Py3.0, the "and" and "or" operators be simplified to
always return a Boolean value instead of returning the last evaluated
argument.

1) The construct can be error-prone. When an error occurs it can be
invisible to the person who wrote it. I got bitten in published code
that had survived testing and code review:

def real(self):
'Return a vector with the real part of each input element'
# do not convert integer inputs to floats
return self.map(lambda z: type(z)==types. ComplexType and z.real or z)

The code fails silently when z is (0+4i). It took a good while to trace
down a user reported error (when Matlab results disagreed with my matrix
module results) and determine that the real() method contained an error.
Even when traced down, I found it hard to see the error in the code.
Now that I know what to look for, it has not happened again, but I do
always have to stare hard at any "and/or" group to mentally verify each
case.
[end quote]
Dare I suggest that if Raymond wasn't struggling to force the body of his
function real() to be a one-liner, he wouldn't have tried to use the "x
and y or z" syntax that bit him? Brevity is not always a virtue.
--
Steven.

[bonono]

Steven D'Aprano wrote:

Why do you assume that everything you need for your list comprehension has
to go into a single line? Chances are your list comp already calls
functions, so just create one more for it to use.
py> def describe(cond):
... if cond:
... return "odd"
... else:
... return "even"
...
py> L = [describe(n % 2) for n in range(8)]
py> L
['even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd']
this makes me "jump" from the list comprehension to the function,
harder to follow than 'in place', especially when the function is
simple, it is not necessary. What is the reason for introducing list
comprehension and then later, generator expression when :

a=[]
for x in range(8):
a.append(descri be(x%2))
return a

and

for x in range(8):
yield describe(x%2)

can do the same thing ?

Doesn't it violate the general idiom that it better has one and only
one way to do certain thing ? I believe part of the reason is that one
doesn't need to "jump" up and down.

One major advantage is that this makes it easier to test your function
describe() in isolation, always a good thing.

Another advantage is that the idiom "call a function" is extensible to
more complex problems:

def describe(n):
if n < 0:
return "negative " + describe(-n)
elif n == 0:
return "zero"
elif n % 2:
return "odd"
else:
return "even"

L = [describe(n) for n in range(8)]

if much easier to understand and follow than using ternary expressions:

# obviously untested
L = ["zero" if n == 0 else \
"negative " + ("odd" if n % 2 else "even") if n < 0 else \
"odd" if n % 2 else "even" for n in range(8)]

Yes, I've seen ternary expressions nested three and even four deep.

Now that is obsession for(and bad use of) one-liner. But picking a bad
example of one liner to rule out its effectiveness and readability is a
stretch I would say.

[bonono]

Steven D'Aprano wrote:

Why do you assume that everything you need for your list comprehension has
to go into a single line? Chances are your list comp already calls
functions, so just create one more for it to use.
py> def describe(cond):
... if cond:
... return "odd"
... else:
... return "even"
...
py> L = [describe(n % 2) for n in range(8)]
py> L
['even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd']
this makes me "jump" from the list comprehension to the function,
harder to follow than 'in place', especially when the function is
simple, it is not necessary. What is the reason for introducing list
comprehension and then later, generator expression when :

a=[]
for x in range(8):
a.append(descri be(x%2))
return a

and

for x in range(8):
yield describe(x%2)

can do the same thing ?

Doesn't it violate the general idiom that it better has one and only
one way to do certain thing ? I believe part of the reason is that one
doesn't need to "jump" up and down.

One major advantage is that this makes it easier to test your function
describe() in isolation, always a good thing.

Another advantage is that the idiom "call a function" is extensible to
more complex problems:

def describe(n):
if n < 0:
return "negative " + describe(-n)
elif n == 0:
return "zero"
elif n % 2:
return "odd"
else:
return "even"

L = [describe(n) for n in range(8)]

if much easier to understand and follow than using ternary expressions:

# obviously untested
L = ["zero" if n == 0 else \
"negative " + ("odd" if n % 2 else "even") if n < 0 else \
"odd" if n % 2 else "even" for n in range(8)]

Yes, I've seen ternary expressions nested three and even four deep.

Now that is obsession for(and bad use of) one-liner. But picking a bad
example of one liner to rule out its effectiveness and readability is a
stretch I would say.

[bonono]

Steven D'Aprano wrote:

L = ["zero" if n == 0 else \
"negative " + ("odd" if n % 2 else "even") if n < 0 else \
"odd" if n % 2 else "even" for n in range(8)]

BTW, the continuation is not necessary I believe.

[ x==0 and "zero" or ["","-"][x < 0] + ("even", "odd")[x%2] for x in
range(8) ]

isn't too bad. though I like the C one a bit more(as it doesn't
overload the and/or)

[ x==0 ? "zero" : ["","-"][x < 0] + ("even", "odd")[x%2] for x in
range(8) ]

As it is still a very straight forward flow(from left to right, without
"internal decision split in between").

[bonono]

Steven D'Aprano wrote:

L = ["zero" if n == 0 else \
"negative " + ("odd" if n % 2 else "even") if n < 0 else \
"odd" if n % 2 else "even" for n in range(8)]

BTW, the continuation is not necessary I believe.

[ x==0 and "zero" or ["","-"][x < 0] + ("even", "odd")[x%2] for x in
range(8) ]

isn't too bad. though I like the C one a bit more(as it doesn't
overload the and/or)

[ x==0 ? "zero" : ["","-"][x < 0] + ("even", "odd")[x%2] for x in
range(8) ]

As it is still a very straight forward flow(from left to right, without
"internal decision split in between").

[Daniel Crespo]

> WHY WHY WHY the obsession with one-liners? What is wrong with the good old

fashioned way?
if cond:
x = true_value
else:
x = false_value

Let me tell you something: I'm not a one-liner coder, but sometimes It
is necesary.
For example:
I need to translate data from a DataField to Another.

def Evaluate(condit ion,truepart,fa lsepart):
if condition:
return truepart
else:
return falsepart

dOldDataFields = {}
dNewDataFields = {}

dNewDataFields = {
'CODE': dOldDataFields['CODEDATA'],
'DATE': dOldDataFields['DATE'],
'CONTACT': Evaluate(dOldDa taFields['CONTACTTYPE']==2,
dOldDataFields['FIRSTCONTACT'], dOldDataFields['SECONDCONTACT'])
}

With this, I created a new dic very easy, saving in
dNewDataFields['CONTACT'] the value of dOldDataFields['FIRSTCONTACT']
or the value of dOldDataFields['SECONDCONTACT'] depending on
dOldDataFields['CONTACTTYPE']. How you do this in a practic way without
the use of one-line code? It is needed! You can't avoid it! Even using
a = [if_false_expr, if_true_expr][predicate] or a function, you'll
always have to use a one-line code (for this purpose, of course).

Daniel

[Luis M. Gonzalez]

This could be done easier this way:

L = [('odd','even')[n%2] for i in range(8)]

[Luis M. Gonzalez]

This could be done easier this way:

L = [('even','odd')[n%2] for n in range(8)]

[bonono]

Luis M. Gonzalez wrote:

This could be done easier this way:

L = [('even','odd')[n%2] for n in range(8)]

That is even/odd, his(created to demonstrate the uglies of ternary) has
4 states, zero, -/+ then even/odd.

[Fredrik Lundh]

Daniel Crespo wrote:

Let me tell you something: I'm not a one-liner coder, but sometimes It
is necesary.
For example:
I need to translate data from a DataField to Another.

def Evaluate(condit ion,truepart,fa lsepart):
if condition:
return truepart
else:
return falsepart

dOldDataFields = {}
dNewDataFields = {}

dNewDataFields = {
'CODE': dOldDataFields['CODEDATA'],
'DATE': dOldDataFields['DATE'],
'CONTACT': Evaluate(dOldDa taFields['CONTACTTYPE']==2,
dOldDataFields['FIRSTCONTACT'], dOldDataFields['SECONDCONTACT'])
}

With this, I created a new dic very easy, saving in
dNewDataFields['CONTACT'] the value of dOldDataFields['FIRSTCONTACT']
or the value of dOldDataFields['SECONDCONTACT'] depending on
dOldDataFields['CONTACTTYPE']. How you do this in a practic way without
the use of one-line code? It is needed! You can't avoid it!

if you use less verbose names, you can do the same thing in less than half
the number of characters, without a single oneliner:

def convert(old):

new = dict(
CODE=old['CODEDATA'],
DATE=old['DATE']
)

if old['CONTACTTYPE'] == 2:
new['CONTACT'] = old['FIRSTCONTACT']
else:
new['CONTACT'] = old['SECONDCONTACT']

return new

</F>