How do I fill a column with one value in Pandas?

The good answer above throws a warning. You can also do:

df.insert(0, 'A', 'foo')

where 0 is the index where the new column will be inserted.

Just select the column and assign like normal:

In [194]:
df['A'] = 'foo'
df

Out[194]:
     A
0  foo
1  foo
2  foo
3  foo

Assigning a scalar value will set all the rows to the same scalar value

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