gh-156680: Raise the documented error from IPv6Network.next_network() by fedonman · Pull Request #156681 · python/cpython

fedonman · 2026-08-30T19:40:41Z

Range-check next_ip against _ALL_ONES in next_network() instead of catching OverflowError, which only the IPv4 path raises. The IPv6 path went through _BaseV6._string_from_ip_int(), whose ValueError escaped uncaught.

next_network() is new in 3.16 and unreleased, so this is folded into the entry the method landed with and needs no NEWS fragment.

testNextNetworkOutOfAddressSpace (test.test_ipaddress.IpaddrUnitTest.testNextNetworkOutOfAddressSpace) ... ok
Total tests: run=1 (filtered)
Result: SUCCESS
Total tests: run=215
Result: SUCCESS

Issue: IPv6Network.next_network() raises the wrong error when addresses run out #156680

…work() next_network() guarded address-space exhaustion with except OverflowError, which only int.to_bytes() on the IPv4 path raises. _BaseV6._string_from_ip_int() raises ValueError instead, so the handler never ran for IPv6 and the internal 'IPv6 address is too large' message escaped. Range-check next_ip against _ALL_ONES before formatting it, which decides the outcome for both address families before either path runs.

StanFromIreland · 2026-08-31T09:52:46Z

Thanks! Can you also please fix the What's New entry (and news entry) for these:

StanFromIreland · 2026-08-31T09:55:19Z

Also, reviewing fadb785 please fix the docstring of next_network, the sentence detailing the arguments is missing a period and there's no raises section.

StanFromIreland · 2026-08-31T10:07:45Z

Oh and one more thing. I don't quite understand why we're doing this little dance with _string_from_ip_int, when we can create an instance directly.

Apologies for all the changes I've asked you to make, indeed it grew to be quite a long list. As such, here's a patch instead:

--- a/Lib/ipaddress.py
+++ b/Lib/ipaddress.py
@@ -1124,11 +1124,15 @@ def next_network(self, next_prefix=None):
 
         Args:
             next_prefix: The desired next prefix length, if not specified the
-            same self.prefixlen will be used
+            same self.prefixlen will be used.
 
         Returns:
             An IPv(4|6) Network object of the next closest network.
 
+        Raises:
+            ValueError: If next_prefix is outside the range of valid prefix
+            lengths, or if no further network of that size exists.
+
         """
         if next_prefix is None:
             next_prefix = self.prefixlen
@@ -1150,15 +1154,13 @@ def next_network(self, next_prefix=None):
             ((new_netmask._ip & self.network_address._ip) >> bit_shift) + 1
         ) << bit_shift
 
-        try:
-            return self.__class__(
-                f"{self._string_from_ip_int(next_ip)}/{next_prefix}"
-            )
-        except OverflowError:
+        if next_ip > self._ALL_ONES:
             raise ValueError(
                 f"out of address space, cannot make another /{next_prefix} "
                 "network"
-            ) from None
+            )
+
+        return self.__class__((next_ip, next_prefix))

…network-ipv6

Finish the next_network() docstring: a period on the argument sentence and a Raises section for the two ValueError cases. Build the result from an (address, prefix) tuple rather than formatting and reparsing a string. Give the What's New and NEWS entries explicit link titles so the IPv4Network and IPv6Network methods no longer both render as next_network().

read-the-docs-community · 2026-09-04T14:50:36Z

Documentation build overview

📚 cpython-previews | 🛠️ Build #34394344 | 📁 Comparing e9745c3 against main (1860130)

🔍 Preview build

2 files changed

± whatsnew/3.16.html
± whatsnew/changelog.html

fedonman · 2026-09-04T15:47:26Z

@StanFromIreland Thanks for the comments! Implemented all.

StanFromIreland

Thanks, this looks good to me. @orsenthil can you please take a look as well?

orsenthil · 2026-09-09T23:37:27Z

-            ) from None
+            )
+
+        return self.__class__((next_ip, next_prefix))


While technically this style can be used self.__class__((next_ip, next_prefix)) m because IPv4Network and IPv6Network do the _split_addr_prefix, we do not advertise or mention about this the value of the address argument. This styled tripped me a bit.

I would prefer the previous style.

return self.__class__( f"{self._string_from_ip_int(next_ip)}/{next_prefix}" )

Because of what we say address can be in the doc strings of the class.

I realize that public docs document that.
https://docs.python.org/3/library/ipaddress.html#ipaddress.IPv4Network
https://docs.python.org/3/library/ipaddress.html#ipaddress.IPv6Network

The change might simply be a follow up doc string patch on here.

cpython/Lib/ipaddress.py

Lines 2301 to 2319 in a6d25db

address: A string or integer representing the IPv6 network or the

IP and prefix/netmask.

'2001:db8::/128'

'2001:db8:0000:0000:0000:0000:0000:0000/128'

'2001:db8::'

are all functionally the same in IPv6. That is to say,

failing to provide a subnetmask will create an object with

a mask of /128.

Additionally, an integer can be passed, so

IPv6Network('2001:db8::') ==

IPv6Network(42540766411282592856903984951653826560)

or, more generally

IPv6Network(int(IPv6Network('2001:db8::'))) ==

IPv6Network('2001:db8::')

strict: A boolean. If true, ensure that we have been passed

A true network address, eg, 2001:db8::1000/124 and not an

IP address on a network, eg, 2001:db8::1/124.

orsenthil

LGTM

orsenthil · 2026-09-10T00:07:09Z

Thanks for the patch @fedonman and review and ping @StanFromIreland .

orsenthil · 2026-09-10T19:30:18Z

@StanFromIreland - could you please review this follow up PR #157280

…work() (python#156681) * pythongh-156680: Raise the documented error from IPv6Network.next_network() next_network() guarded address-space exhaustion with except OverflowError, which only int.to_bytes() on the IPv4 path raises. _BaseV6._string_from_ip_int() raises ValueError instead, so the handler never ran for IPv6 and the internal 'IPv6 address is too large' message escaped. Build the result from an (address, prefix) tuple rather than formatting and reparsing a string.

bedevere-app Bot mentioned this pull request Aug 30, 2026

IPv6Network.next_network() raises the wrong error when addresses run out #156680

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bedevere-app Bot added the awaiting review label Aug 30, 2026

fedonman mentioned this pull request Aug 30, 2026

gh-87027: Add prev_network function to ipaddress library (follow-up to GH-24180) #154041

Open

StanFromIreland added the skip news label Aug 31, 2026

StanFromIreland requested a review from orsenthil August 31, 2026 09:59

fedonman added 2 commits September 4, 2026 17:44

Merge remote-tracking branch 'upstream/main' into fix-ipaddress-next-…

43884d5

…network-ipv6

fedonman requested a review from AA-Turner as a code owner September 4, 2026 14:45

StanFromIreland approved these changes Sep 4, 2026

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bedevere-app Bot added awaiting merge and removed awaiting review labels Sep 4, 2026

orsenthil reviewed Sep 9, 2026

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orsenthil approved these changes Sep 10, 2026

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orsenthil merged commit 69a6612 into python:main Sep 10, 2026
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bedevere-app Bot removed the awaiting merge label Sep 10, 2026

orsenthil mentioned this pull request Sep 10, 2026

gh-156680 - Update IPv4Network and IPv6Network docstring. #157280

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gh-156680: Raise the documented error from IPv6Network.next_network() - #156681

gh-156680: Raise the documented error from IPv6Network.next_network()#156681
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fedonman:fix-ipaddress-next-network-ipv6

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	address: A string or integer representing the IPv6 network or the
	IP and prefix/netmask.
	'2001:db8::/128'
	'2001:db8:0000:0000:0000:0000:0000:0000/128'
	'2001:db8::'
	are all functionally the same in IPv6. That is to say,
	failing to provide a subnetmask will create an object with
	a mask of /128.

	Additionally, an integer can be passed, so
	IPv6Network('2001:db8::') ==
	IPv6Network(42540766411282592856903984951653826560)
	or, more generally
	IPv6Network(int(IPv6Network('2001:db8::'))) ==
	IPv6Network('2001:db8::')

	strict: A boolean. If true, ensure that we have been passed
	A true network address, eg, 2001:db8::1000/124 and not an
	IP address on a network, eg, 2001:db8::1/124.

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