<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_transactions(self, include_investment=False):
 """Returns the transaction data as a Pandas DataFrame."""

        assert_pd()
        s = StringIO(self.get_transactions_csv(
            include_investment=include_investment))
        s.seek(0)
        df = pd.read_csv(s, parse_dates=['Date'])
        df.columns = [c.lower().replace(' ', '_') for c in df.columns]
        df.category = (df.category.str.lower()
                       .replace('uncategorized', pd.np.nan))
        return df