Fix Route handlers convert falsy values (0, False) to empty strings #572

[civvic]

Fix route handlers converting falsy values (0, False) to empty strings (#572)

Problem

Route handlers returning 0 or False rendered as an empty string instead of '0' / 'False'. Values like 1 and True worked correctly, making the behaviour inconsistent.

Cause

In _resp (fasthtml/core.py), the guard if not resp: resp='' is true for all falsy values. It's meant to catch "no response", but 0 and False are also falsy, so they were silently replaced with '' before reaching the str(resp) conversion lower down.

Fix

Narrow the check to only the value that actually means "no response":

-    if not resp: resp=''
+    if resp is None: resp=''

Now 0 and False fall through to resp = str(resp) and render as '0' and 'False'. None still renders empty, and empty containers ((), []) are unaffected since they were never caught by this line — they flow through the FT-rendering path as before.

Testing

@rt('/zero')
def get(): return 0

assert cli.get('/zero').text == '0'
assert cli.get('/false').text == 'False'
assert cli.get('/one').text == '1'
assert cli.get('/true').text == 'True'

Closes #572.

[jph00]

Thanks :)