Pure-python implementation:
    if isinstance(o, (list, tuple)):

C implementation:
    if (PyList_Check(obj) || PyTuple_Check(obj))

This make real difference (!) in my code.

So, please change pure-python implementation to:
    if type(o) in (list, tuple):
Or, fix C implementation to: /* intentionally forgot (only for this example) to check if return value is -1 */
    if (PyObject_IsInstance(obj, PyList_Type) || PyObject_IsInstance(obj, PyTuple_Type)

What difference does it make? Are you using __instancecheck__ perhaps?

#!/usr/bin/python2.7

import json

class pseudo_list(object):
    __class__ = list # fake isinstance

    def __init__(self, iterator):
        self._saved_iterator = iterator

    def __iter__(self):
        return self._saved_iterator

class myenc(json.JSONEncoder):
    def default(self, o):
        try:
            return pseudo_list(iter(o))
        except TypeError:
            return super(myenc, self).default(o)

# works (pure-python implementation)
print json.dumps({1:xrange(10), 2:[5,6,7,8]}, cls=myenc, indent=1)

# does not work (C implementation)
print json.dumps({1:xrange(10), 2:[5,6,7,8]}, cls=myenc, indent=None)

> class pseudo_list(object):
>     __class__ = list # fake isinstance

Why not inherit from list directly?
Setting __class__ to something else isn't widely supported in the Python code base. It may work or may not work, depending on the API, but it's not something we design or test for.

Well, __class_ = list is my problem, but python's problem is that it uses different approaches in C and python implementation.

P.S.
I don't want to subclass list, as I don't want things like this:
x = pseudo_list(iter(xrange(10))
x.append('test')
print len(x)

> Well, __class_ = list is my problem, but python's problem is that it
> uses different approaches in C and python implementation.

Well, by construction a C accelerator will use the fastest method
available within what the API's specification allows. The json API
doesn't specify whether isinstance() or a more concrete type check is
used when dispatching over argument types, so I'd classify this as an
implementation detail.

Inconsistency is bother me. If I specify indent in dumps(), I will have one semantics, else other ones.

Why not to fix pure-python implementation using "type(o) in (list, tuple)" ? This is faster too (as I think).

FWIW, I think Mark's right here. I'm +1 on the implementations being consistent.

Seems like a potentially nasty surprise if you move from one implementation to the other and, lacking awareness of this quirk, design your algorithm around semantics. I think this was Mark's original point.

If the json API doesn't care how the type check is performed, then we get a (probably very small :)) win from the type(o) in (list, tuple) for the Python impl in addition to bringing consistency to the two implementations.

We have received a notification about this bug for 3.5

FWIW, the C implementation of the sequence encoder uses PySequence_Fast(), so adding a lower priority instance check that calls the same encoding function would solve this.

https://github.com/python/cpython/blob/cfa797c0681b7fef47cf93955fd06b54ddd09a7f/Modules/_json.c#L1730

Probably not something to fix in Py3.5/6 anymore, though.