IOError: [Errno 2] No such file or directory: 'learning_python.txt'

10-1练习  
```
--- Reading in the entire file:
Traceback (most recent call last):
  File "./Python-Crash-Course-Homework/chapter10/10-1.py", line 7, in <module>
    with open(filename) as f:
IOError: [Errno 2] No such file or directory: 'learning_python.txt'
```

文件确实存在，文件名没有写错，路径正确，是哪里出了问题？