Why does the pandas the dataframe column order change automatically？

Question

When I were output the result to CSV file, I generated a pandas dataframe. But the dataframe column order changed automatically, I am curious Why would this happened?

Answer 1

As Youn Elan pointed out, python dictionaries aren't ordered, so if you use a dictionary to provide your data, the columns will end up randomly ordered. You can use the columns argument to set the order of the columns explicitly though:

import pandas as pd

before = pd.DataFrame({'lake_id': range(3), 'area': (['a', 'b', 'c'])})
print 'before'
print before

after = pd.DataFrame({'lake_id': range(3), 'area': (['a', 'b', 'c'])},
                     columns=['lake_id', 'area'])
print 'after'
print after

Result:

before
  area  lake_id
0    a        0
1    b        1
2    c        2
after
   lake_id area
0        0    a
1        1    b
2        2    c

Answer 2

I notice you use a dictionary.

Dictionaries in python are not garanteed to be in any order. It depends on multiple factors, including what's in the array. Keys are garanteed to be unique though