Ternary operators in list comprehensions
a = [1,2,3] [ x for x in a if x & 1] [1, 3] [ x for x in a if x & 1 else 'even'] File "<stdin>", line 1 [ x for x in a if x & 1 else 'even'] ^ SyntaxError: invalid syntax
I expected [1, 'even', 3] I would expect that the if expression would be able to provide alternative values through else. The work around blows it out to: l = [] for x in a: if x&1: l.append(x) else: l.append('even') Unless there is a better way?
[x if x & 1 else 'even' for x in a] An `if` at the end of the comprehension means a condition on whether to include the value. Also, this question would have been better asked on python-list. 2017-10-05 8:40 GMT-07:00 Jason H <jhihn@gmx.com>:
a = [1,2,3] [ x for x in a if x & 1] [1, 3] [ x for x in a if x & 1 else 'even'] File "<stdin>", line 1 [ x for x in a if x & 1 else 'even'] ^ SyntaxError: invalid syntax
I expected [1, 'even', 3]
I would expect that the if expression would be able to provide alternative values through else.
The work around blows it out to: l = [] for x in a: if x&1: l.append(x) else: l.append('even')
Unless there is a better way? _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
a = [1,2,3] [x if x & 1 else 'even' for x in a] [1, 'even', 3]
You're mixing the if clause of the list comprehension up with a ternary expresssion. There's no "else" in the list comprehension if clause. Paul On 5 October 2017 at 16:40, Jason H <jhihn@gmx.com> wrote:
a = [1,2,3] [ x for x in a if x & 1] [1, 3] [ x for x in a if x & 1 else 'even'] File "<stdin>", line 1 [ x for x in a if x & 1 else 'even'] ^ SyntaxError: invalid syntax
I expected [1, 'even', 3]
I would expect that the if expression would be able to provide alternative values through else.
The work around blows it out to: l = [] for x in a: if x&1: l.append(x) else: l.append('even')
Unless there is a better way? _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Putting it another way, your example doesn't make sense. How would you parenthesise it to make it clearer? [ (x for x in a if x & 1) else 'even'] You have an "else" without an "if". [ x for x in (a if x & 1 else 'even')] Using x before it has been defined, at least in this line of code. [ (x for x in a) if x & 1 else 'even'] Ditto Other variants may be possible. Whereas Jelle's correct version can be written as [(x if x & 1 else 'even') for x in a] [1, 'even', 3] Rob Cliffe On 05/10/2017 16:44, Jelle Zijlstra wrote:
[x if x & 1 else 'even' for x in a]
An `if` at the end of the comprehension means a condition on whether to include the value.
Also, this question would have been better asked on python-list.
2017-10-05 8:40 GMT-07:00 Jason H <jhihn@gmxcom <mailto:jhihn@gmx.com>>:
>>> a = [1,2,3] >>> [ x for x in a if x & 1] [1, 3] >>> [ x for x in a if x & 1 else 'even'] File "<stdin>", line 1 [ x for x in a if x & 1 else 'even'] ^ SyntaxError: invalid syntax
I expected [1, 'even', 3]
I would expect that the if expression would be able to provide alternative values through else.
The work around blows it out to: l = [] for x in a: if x&1: l.append(x) else: l.append('even')
Unless there is a better way? _______________________________________________ Python-ideas mailing list Python-ideas@python.org <mailto:Python-ideas@python.org> https://mail.python.org/mailman/listinfo/python-ideas <https://mail.python.org/mailman/listinfo/python-ideas> Code of Conduct: http://python.org/psf/codeofconduct/ <http://python.org/psf/codeofconduct/>
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On Thu, Oct 05, 2017 at 05:40:32PM +0200, Jason H wrote:
a = [1,2,3] [ x for x in a if x & 1] [1, 3] [ x for x in a if x & 1 else 'even'] File "<stdin>", line 1 [ x for x in a if x & 1 else 'even'] ^ SyntaxError: invalid syntax
[(x if x & 1 else 'even') for x in a] The if clause in the list comprehension determines which items are included, and there is no "else" allowed. You don't want to skip any values: the list comp should have the same number of items as "a" (namely, 3) the comprehension if clause is inappropriate. Instead, you want the value in the comprehension to conditionally depend on x. By the way, this list is for suggesting improvements and new functionality to the language, not for asking for help. You should consider asking your questions on python-list or tutor mailing lists instead. -- Steve
participants (5)
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Jason H -
Jelle Zijlstra -
Paul Moore -
Rob Cliffe -
Steven D'Aprano