Difference between binomial, binomial() and 'binomial' - r

Question

What is the difference between binomial, binomial() and 'binomial' when using glm. They are not identical, as can be see by following code:
> library(MASS)
> bwdf = birthwt[-10]
> mod = glm(low~., data=bwdf, family=binomial)
> mod2 = glm(low~., data=bwdf, family=binomial())
> mod3 = glm(low~., data=bwdf, family="binomial")
> identical(mod, mod2)
[1] FALSE
> identical(mod3, mod2)
[1] FALSE
> identical(mod3, mod)
[1] FALSE
But the values are identical:
> mod
Call: glm(formula = low ~ ., family = binomial, data = bwdf)
Coefficients:
(Intercept) age lwt race2 race3 smoke1 ptl ht1 ui1 ftv
0.48062 -0.02955 -0.01542 1.27226 0.88050 0.93885 0.54334 1.86330 0.76765 0.06530
Degrees of Freedom: 188 Total (i.e. Null); 179 Residual
Null Deviance: 234.7
Residual Deviance: 201.3 AIC: 221.3
>
> mod2
Call: glm(formula = low ~ ., family = binomial(), data = bwdf)
Coefficients:
(Intercept) age lwt race2 race3 smoke1 ptl ht1 ui1 ftv
0.48062 -0.02955 -0.01542 1.27226 0.88050 0.93885 0.54334 1.86330 0.76765 0.06530
Degrees of Freedom: 188 Total (i.e. Null); 179 Residual
Null Deviance: 234.7
Residual Deviance: 201.3 AIC: 221.3
>
> mod3
Call: glm(formula = low ~ ., family = "binomial", data = bwdf)
Coefficients:
(Intercept) age lwt race2 race3 smoke1 ptl ht1 ui1 ftv
0.48062 -0.02955 -0.01542 1.27226 0.88050 0.93885 0.54334 1.86330 0.76765 0.06530
Degrees of Freedom: 188 Total (i.e. Null); 179 Residual
Null Deviance: 234.7
Residual Deviance: 201.3 AIC: 221.3
Is there any difference?

Answer 1

Remember that the identical function is very picky and that part of your mod objects is the call that was used to create the object. That call piece will differ based on the parentheses and quotes and so identical will say that they differ. Try calling identical on the pieces of the mod objects that you care about and see if they are identical.
If you look at the first few lines of the code of glm you will see that it checks the family argument and if it is a character string, then it uses get to "get" the function of that name. If family is a function (either passed in, or as a result of get) then it calls the function. So whether you pass in the name as a character string, the function, or the results of evaluating the function, after the 1st part of the code you will have the exact same thing in family and therefore the same results (but the call will be different).