mutate() discards user-defined attributes

[choisy]

The functions filter() and mutate() behave differently in what regards user-defined attributes.
Below is an example. Let's start by adding an attribute to a data frame:

attr(mtcars, "my_attribute_name") <- "my_attribute_value"

We can see the attributes of this data frame:

attributes(mtcars)

The function filter() behaves as expected in the sense that it does not affect the attributes of the data frame (apart from row.names of course):

mtcars %>%
  filter(mpg > 20) %>%
  attributes()

On the other hand, the function mutate() does affect the attributes of the data. Specifically, the above user-defined attribute is discarded (the other ones are unaffected):

mtcars %>%
  mutate(a_new_variable = 3) %>%
  attributes()

I think that it would be nicer if mutate() preserved the user-defined attributes as filter() does.

[hadley]

Duplicate of #3429

[lock]

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