Increase index of pandas DataFrame by one

Question

I'd like to have my dataframe start with index 1 instead of 0. But somehow I am not getting it:

In[1]: df = pd.DataFrame([[4,7],[10,11],[7,2]],columns=['one', 'two'])
In[2]: df
Out[2]: 
   one  two
0    4    7
1   10   11
2    7    2
In[3]: df.reindex(range(1,len(df)+1))
Out[3]: 
   one  two
1   10   11
2    7    2
3  NaN  NaN

Where did my first row go? What am I getting wrong about reindex()?

Answer 1

The reindexing does not reassign the index values and preserve the order for that you can assign directly:

In [25]:

df.index = range(1,len(df)+1)
df
Out[25]:
   one  two
1    4    7
2   10   11
3    7    2

The docs show that you are conforming your data to the new index which will introduce NaN values where none existed hence why you lost a row, this is why there is a fillna param for reindex.

Answer 2

To increase your index with 1 you can simply modify the index like this, df.index += 1.

Full example:

In [1]: import pandas as pd

In [2]: df = pd.DataFrame([[4,7],[10,11],[7,2]],columns=['one', 'two'])

In [3]: df
Out[3]:
   one  two
0    4    7
1   10   11
2    7    2

In [4]: df.index += 1

In [5]: df
Out[5]:
   one  two
1    4    7
2   10   11
3    7    2