>>> import os, sys
>>> os.dirname()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'dirname'
>>> sys.dirname()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'dirname'
>>> dirname()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'dirname' is not defined
>>> help(dirname)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'dirname' is not defined
>>>

>>> s = "/home/vasya/test/test.txt"
>>> s = func(s, "re_")
>>> s
'/home/vasya/test/re_test.txt'
>>> s = func(s, "re_", 're_')
>>> s
'/home/vasya/test/re_re_test.txt'
>>> s = func(s, "__1__", 'test')
>>> s
'/home/vasya/test/re_re_test__1__.txt'
>>> func("abc", "re_")
'abc'
>>> func("abc", "re_", "a")
'are_bc'
>>>

>>> def func(path, prefix, delim='/'):
...   if not delim in path:
...     return prefix + path
...   return path.rsplit(delim, 1)[0] + delim + prefix + path.rsplit(delim, 1)[-1]
...
>>> def func2(path, prefix):
...   import os
...   return os.path.join(os.path.dirname(path), prefix + os.path.basename(path))
...
>>> def form():
...   l = []
...   for i in range(1, 10001):
...     l.append("/home/guest/file%d.txt" % i)
...   return l
...
>>> def f1():
...   outcome = []
...   l = form()
...   for p in l:
...     outcome.append(func(p, "prefix_"))
...   return outcome
...
>>> def f2():
...   outcome = []
...   l = form()
...   for p in l:
...     outcome.append(func2(p, "prefix_"))
...   return outcome
...
>>> from timeit import Timer as reg
>>> t1 = reg(f1)
>>> t2 = reg(f2)
>>> t1.repeat(3, 1)
[0.06797289848327637, 0.04929399490356445, 0.05050206184387207]
>>> t2.repeat(3, 1)
[0.15523314476013184, 0.1336510181427002, 0.13914895057678223]
>>>