short-circuit evaluation

[sky-cake]

Does this have any other names besides short-circuit evaluation?

show_warning = False
warning = t'<div class="alert">Warning message</div>'
page = html(t"<main>{show_warning and warning}</main>")
# <main></main>

It seems to involve more than the traditional meaning of that term. From playing around, it seems like the rightmost Template value is always used. So it's more like "short-circuit rightmost value usage".

>>> show_warning = t'<b>False</b>'
... warning = t'<div class="alert">Warning message</div>'
... page = html(t"<main>{show_warning and show_warning and warning}</main>")
... print(page)
... 
<main><div class="alert">Warning message</div></main>

>>> show_warning = t'<b>False</b>'
... page = html(t"<main>{show_warning and show_warning}</main>")
... print(page)
... 
<main><b>False</b></main>

>>> show_warning = 'omg'
... page = html(t"<main>{show_warning and show_warning}</main>")
... print(page)
... 
<main>omg</main>

[davepeck]

Does this have any other names besides short-circuit evaluation?

Not that I'm aware of!

There's no magic in this syntax: since you're inside curly braces, it's just Python. Python has short circuit boolean operators. In addition to handling boolean values (True and False), they handle "truth-y" values (non-empty strings, non-zero integers, and anything else for which bool(value) is True) and "false-y" values (empty strings, 0, anything for which bool(value) is False).

and evaluates each value in turn. If one of them is false-y, and returns it right away (that's the short circuit). Otherwise, and returns the last evaluated value, which will be on the right.

A few examples that just use Python, not tdom:

>>> False and "hello"
False
>>> True and "hello"
"hello"
>>> "" and "hello"  # "" is considered false-y
""
>>> "fun" and "hello"  # "fun" is truth-y
"hello"
>>> "fun" and "hello" and "cool"
"cool"
>>> "fun" and 0 and "cool"
0

Likewise, or evaluates each value in turn. If one of them is truth-y, or returns it right away (that's the short circuit). Otherwise, or returns the last evaluated value, which will be on the right.

>>> False or "hello"
"hello"
>>> True or "hello"
True
>>> False or ""
""

[sky-cake]

Thanks for pointing this out, Dave