dataframe works if I take data from another dataframe but not if add it directly

Question

I am hoping someone can help me understand why a dataframe works one way but not the other. I am relatively new to Python but do have a decent understanding of Pandas. However I can't figure out why I can't add data to the empty dataframe directly first without getting data from another dataframe. I know there are plenty of ways around this. I am just curious as to why.

Thank you for any light you may be able to shed on this.

import pandas as pd
    
source_df = pd.DataFrame({'iset':['1001']})
print(source_df)

column_names = ['Import Set No','col2']  
df = pd.DataFrame(columns = column_names)  

df.loc[:,'col2'] = 2
df.loc[:,'Import Set No'] = source_df.loc[:,'iset']

print(df)

Produces:

  iset
0  1001
Empty DataFrame
Columns: [Import Set No, col2]
Index: []

And:

import pandas as pd
   
source_df = pd.DataFrame({'iset':['1001']})
print(source_df)

column_names = ['Import Set No','col2']    
df = pd.DataFrame(columns = column_names)
   
df.loc[:,'Import Set No'] = source_df.loc[:,'iset']
df.loc[:,'col2'] = 2
  
print(df)

Produces:

   iset
0  1001
Index(['Import Set No', 'col2'], dtype='object')
  Import Set No  col2
0          1001     2