Python TypeError: cannot concatenate object of type '<class 'str'>'; only Series and DataFrame objs are valid

I am getting the following error:

TypeError: cannot concatenate object of type '<class 'str'>'; only Series and DataFrame objs are valid 
Traceback (most recent call last):
  File "/var/task/etd.py", line 77, in lambda_handler
    data = pd.concat(concated_data, ignore_index=True)

In the following block:

for idx, day_row in curr_df.iterrows():
    av_last_month, rate_end_last_mon = last_months_avs(df_to_date, day_row['From_Currency'], day_row['To_Currency'], day_row['Obs_Date'])

concated_data = ({'Obs_Date': day_row['Obs_Date'], 'From_Currency': day_row['From_Currency'], 
'To_Currency': day_row['To_Currency'], 'Rate': day_row['Rate'],
'PE_Rate': av_last_month, 'Mon_End_Rate': rate_end_last_mon})

data = pd.concat(concated_data, ignore_index=True)

new_df = pd.DataFrame(data)

>Solution :

One of your values is a string and not a Series or a DataFrame:

concated_data = ({'key1': pd.Series(), 'key2': pd.Series()})
pd.concat(concated_data, ignore_index=True)

# Output
Series([], dtype: object)

However:

concated_data = ({'key1': pd.Series(), 'key2': pd.Series(), 'key3': ''})
pd.concat(concated_data, ignore_index=True)

# Output
...
TypeError: cannot concatenate object of type '<class 'str'>'; only Series and DataFrame objs are valid

To debug, try:

for key, val in concated_data.items():
    print(key, type(val))

# Output
key1 <class 'pandas.core.series.Series'>
key2 <class 'pandas.core.series.Series'>
key3 <class 'str'>

Note: I think you should also use axis=1 as argument of pd.concat.