Dataframe, keep only one column

Question

I can't find the pandas function that returns a one column Dataframe from a multi column DF. I need the exact opposit function of the drop(['']) one.

Any ideas?

Answer 1

You can use the following notation to return a single column dataframe:

df = pd.DataFrame(data=np.random.randint(1, 100 ,(10, 5)), columns=list('ABCDE'))

df_out = df[['C']]

Output:

    C
0  65
1  48
2   1
3  41
4  85
5  55
6  45
7  10
8  44
9  11

Note: df['C'] returns a series. And, you can use the to_frame method to convert that series into a dataframe. Or use the double brackets, [[]].

Answer 2

For completeness, I would like to show how we can use the parameter drop to obtain a one column dataframe from a multicolumn one. Also, I explain the result using the tidyverse universe (paper).

Working with a minimal example for a dataframe DF

library(tidyverse)

DF <- data.frame(a = 1:2, b = c("e", "f"))
str(DF)
#> 'data.frame':    2 obs. of  2 variables:
#>  $ a: int  1 2
#>  $ b: chr  "e" "f"

By the way, note that in R versions lower than 4.0, column b would be a factor by default (unless we use stringsAsFactors= FALSE)

Operator [ returns a list (dataframe) as it preserves the original structure (dataframe)

DF[1]
#>   a
#> 1 1
#> 2 2

DF['a']
#>   a
#> 1 1
#> 2 2

On the other hand, operator [[ simplifies the result to the simplest structure possible, a vector for a one-column dataframe. In the three expressions of it, you always get the simplified version (a vector)

DF[[1]]
#> [1] 1 2

DF[['a']]
#> [1] 1 2

DF$a
#> [1] 1 2

Finally, using [ with row and column dimension

DF[, 1]
#> [1] 1 2

also returns the simplified version because the parameter drop is set to TRUE by default. Setting it to FALSE, you preserve the structure and obtain a one-column dataframe

DF[, 1, drop = FALSE]
#>   a
#> 1 1
#> 2 2

A good explanation of this point can be found at: Advanced R by Hadley Wickham, CRC, 2015, section 3.2.1 or section 4.2.5 in the on-line version of the book (June 2021)

Finally, within the tidyverse universe CRAN, you always obtain a dataframe (tibble) when selecting one column

DF %>% 
  select(2)
#>   b
#> 1 e
#> 2 f

DF %>% 
  select("a")
#>   a
#> 1 1
#> 2 2

DF %>% 
  select(a)
#>   a
#> 1 1
#> 2 2

^{Created on 2021-06-04 by the reprex package (v0.3.0)}