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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _build_projection_expression(clean_table_keys):
 """Given cleaned up keys, this will return a projection expression for the dynamodb lookup. Args: clean_table_keys (dict):
 keys without the data types attached Returns: str: A projection expression for the dynamodb lookup. """

    projection_expression = ''
    for key in clean_table_keys[:-1]:
        projection_expression += ('{},').format(key)
    projection_expression += clean_table_keys[-1]
    return projection_expression