Add builtins._ doctest

Defining builtins._ should not break anything outside the doctest while also should be working as expected inside the doctest.

IPython/testing/plugin/test_ipdoctest.py

@@ -1,76 +1,92 b''
 """Tests for the ipdoctest machinery itself.
 
 Note: in a file named test_X, functions whose only test is their docstring (as
 a doctest) and which have no test functionality of their own, should be called
 'doctest_foo' instead of 'test_foo', otherwise they get double-counted (the
 empty function call is counted as a test, which just inflates tests numbers
 artificially).
 """
 
 def doctest_simple():
     """ipdoctest must handle simple inputs
     
     In [1]: 1
     Out[1]: 1
 
     In [2]: print(1)
     1
     """
 
 def doctest_multiline1():
     """The ipdoctest machinery must handle multiline examples gracefully.
 
     In [2]: for i in range(4):
        ...:     print(i)
        ...:      
     0
     1
     2
     3
     """
 
 def doctest_multiline2():
     """Multiline examples that define functions and print output.
 
     In [7]: def f(x):
        ...:     return x+1
        ...: 
 
     In [8]: f(1)
     Out[8]: 2
 
     In [9]: def g(x):
        ...:     print('x is:',x)
        ...:      
 
     In [10]: g(1)
     x is: 1
 
     In [11]: g('hello')
     x is: hello
     """
 
 
 def doctest_multiline3():
     """Multiline examples with blank lines.
 
     In [12]: def h(x):
        ....:     if x>1:
        ....:         return x**2
        ....:     # To leave a blank line in the input, you must mark it
        ....:     # with a comment character:
        ....:     #
        ....:     # otherwise the doctest parser gets confused.
        ....:     else:
        ....:         return -1
        ....:      
 
     In [13]: h(5)
     Out[13]: 25
 
     In [14]: h(1)
     Out[14]: -1
 
     In [15]: h(0)
     Out[15]: -1
    """
+
+
+def doctest_builtin_underscore():
+    """Defining builtins._ should not break anything outside the doctest
+    while also should be working as expected inside the doctest.
+
+    In [1]: import builtins
+
+    In [2]: builtins._ = 42
+
+    In [3]: builtins._
+    Out[3]: 42
+
+    In [4]: _
+    Out[4]: 42
+    """