Exercise for the debugging class

hands_on/bug_hunt/datastore/README

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+This is the directory used as a datastore by `file_datastore.py`.

hands_on/bug_hunt/file_datastore.py

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+import os
+
+
+class FileDatastore:
+    """Datastore based on a regular file system.
+
+    Parameters
+    ----------
+    base_path: str
+        Filesystem path at which the data store is based.
+    """
+
+    def __init__(self, base_path):
+        if not os.path.exists(base_path):
+            raise FileNotFoundError(f'Base path {base_path} does not exist')
+        if not os.path.isdir(base_path):
+            raise NotADirectoryError(f'Base path {base_path} exists but is not a directory')
+
+        self.base_path = base_path
+
+    def open(self, path, mode):
+        """ Open a file-like object
+
+        Parameters
+        ----------
+        path: str
+            Path relative to the root of the data store.
+        mode: str
+            Specifies the mode in which the file is opened.
+
+        Returns
+        -------
+        IO[Any]
+            An open file-like object.
+        """
+        path = os.path.join(self.base_path, path)
+        return open(path, mode)
+
+    def read(self, path):
+        """ Read a sequence of bytes from the data store.
+
+        Parameters
+        ----------
+        path: str
+            Path relative to the root of the data store.
+
+        Returns
+        -------
+        bytes
+            The sequence of bytes read from `path`.
+        """
+        path = os.path.join(self.base_path, path)
+        with self.open(path, 'rb') as f:
+            data = f.read()
+        return data
+
+    def write(self, path, data) -> None:
+        """ Write a sequence of bytes to the data store.
+
+        `path` contains the path relative to the root of the data store, including the name
+        of the file to be created. If a file already exists at `path`, it is overwritten.
+
+        Intermediate directories that do not exist will be created.
+
+        Parameters
+        ----------
+        path: str
+            Path relative to the root of the data store.
+        data: bytes
+            Sequence of bytes to write at `path`.
+        """
+        path = os.path.join(self.base_path, path)
+        self.makedirs(os.path.dirname(path))
+        with self.open(path, 'wb') as f:
+            f.write(data)
+
+    def exists(self, path):
+        """ Returns True if the file exists.
+
+        Parameters
+        ----------
+        path: str
+            Path relative to the root of the data store.
+
+        Returns
+        -------
+        bool
+            True if the file exists, false otherwise
+        """
+        complete_path = os.path.join(self.base_path, path)
+        return os.path.exists(complete_path)
+
+    def makedirs(self, path):
+        """ Creates the specified directory if needed.
+
+        If the directories already exist, the method does not do anything.
+
+        Parameters
+        ----------
+        path: str
+            Path relative to the root of the data store.
+        """
+        complete_path = os.path.join(self.base_path, path)
+        os.makedirs(complete_path, exist_ok=True)
+
+
+if __name__ == '__main__':
+    data = b'A test! 012'
+    datastore = FileDatastore(base_path='./datastore')
+    datastore.write('a/mydata.bin', data)
+
+    # This should pass!
+    # The code works correctly if `base_path` is an absolute path :-(
+    assert os.path.exists('./datastore/a/mydata.bin')