repr() on Tree node causing infinite recursion

Question

Here I have an attempt for implenenting a printable tree:

class Node(object):
    def __init__(self, value, child=[]):
        self.value = value
        self.child = child
    def __repr__(self):
        print('Debug:', self.value)
        valuestr = repr(self.value)
        childstr = ', '.join([repr(x) for x in self.child])
        return "Node({0}, [{1}])".format(valuestr, childstr)
    def __str__(self):
        return self.__repr__()

When I run the following code, it works as expected.

A=Node('A')
B=Node('B')
C=Node('C')
A.child=[B, C]
print(A)

# Output:
# Debug: A
# Debug: B
# Debug: C
# Node('A', [Node('B', []), Node('C' [])])

But, when A.child.append(B) is used instead of A.child=[B, C], it causes an infinite recursion.

Debug: A
Debug: B
Debug: B
Debug: B
...
Debug: B
Traceback (most recent call last):
  File "/storage/emulated/0/com.hipipal.qpyplus/scripts3/.last_tmp.py", line 22, in <module>
    print(A)
  File "/storage/emulated/0/com.hipipal.qpyplus/scripts3/.last_tmp.py", line 15, in __str__
    return self.__repr__()
  File "/storage/emulated/0/com.hipipal.qpyplus/scripts3/.last_tmp.py", line 12, in __repr__
    childstr = ', '.join([repr(x) for x in self.child])
  File "/storage/emulated/0/com.hipipal.qpyplus/scripts3/.last_tmp.py", line 12, in <listcomp>
    childstr = ', '.join([repr(x) for x in self.child])
  File "/storage/emulated/0/com.hipipal.qpyplus/scripts3/.last_tmp.py", line 12, in __repr__
    childstr = ', '.join([repr(x) for x in self.child])
...
RuntimeError: maximum recursion depth exceeded while calling a Python object

And, in case of the following,

A=Node('A')
B=Node('B')
C=Node('C')
A.child.append(B)
print(C)

The same error occurs again; But with Debug: C instead of Debug: A in the first line.

What is causing this behaviour? Why is repr() acting strangely when nodes are appended to the list of children, not assigned?

Currently using Python 3.2.7, qpython for Android.