我遇到过，你这样：
import sys
print sys.path

然后你就明白了，参考：

https://docs.python.org/2/tutorial/modules.html ：

When a module named spam is imported, the interpreter first searches for a built-in module with that name. If not found, it then searches for a file named spam.py in a list of directories given by the variable sys.path. sys.path is initialized from these locations:

the directory containing the input script (or the current directory).
PYTHONPATH (a list of directory names, with the same syntax as the shell variable PATH).
the installation-dependent default.

btw:
你 append 怎么写的？ append src 上一级的绝对或相对路径 应该是没问题的。