What's the difference between using and import in Julia?

Modules can be loaded in Julia using either the using statements or the import statements. While they are often used interchangeably, these statements are different in terms of function extension. In this Answer, we'll discuss the difference between using and import statements.

The using statement

The using statement is conventionally used to import a module. When the using statement is used, the module name is imported in the namespace and the exported variables are available in the global scope.

Example

Note: All code examples in this Answer have been implemented in Julia 1.8.1.

Let's look at the following code example to understand the using statement better:

# Define a dummy variable
module foo
    f = 3
    g = 4
    export f
end
using .foo
println("f: ", f)
println("g: $(foo.g)")

In this code:

  • Lines 2–6: A module foo is defined which defines variables f=3 and g=4 and exports f.

  • Line 8: The module foo is loaded in the main scope using the using statement.

  • Line 9: f is loaded as a global variable because of export in foo, and its value is printed for verification.

  • Line 10: Since g is not exported by foo, it needs to be used with its module name.

Note: If you try to export a variable of the module that's already declared before the module is loaded, Julia will issue a warning and will stick to the prior declaration.

The import statement

When a module is loaded using the import statement, only the module name is loaded in the namespace. All variables and functions need to be loaded using the module name.

Example

We'll use the same example as before, with the key difference of loading the module using the import statement. Let's look at the following code example to understand the import statement better:

# Define a dummy variable
module foo
    f = 3
    g = 4
    export f
end
import .foo
println("f: ", foo.f)
println("g: $(foo.g)")

In this code:

  • Lines 2–6: A module foo is defined which defines variables f=3 and g=4 and exports f.

  • Line 8: The module foo is loaded in the main scope using the import statement.

  • Line 9: Even when f is exported by foo, it can only be used with the module name.

  • Line 10: g is being loaded with its module name.

Method extension

Apart from the scope of the exported variables, one key difference between the using statement and the import statement is that a module method can only be extended using the import statement. Let's have a look at the following code example:

module randomModule
  randomFunction() = "This is a nice function in a nice module."
  export randomFunction
end
using .randomModule: randomFunction
randomFunction() = "This is a nice function."
println(randomFunction())

Here, trying to extend the randomModule.randomFunction() using the using statement causes an error. This can be fixed by using the import statement. 

module randomModule
  randomFunction() = "This is a nice function in a nice module."
  export randomFunction
end
import .randomModule: randomFunction
randomFunction() = "This is a nice function outside the nice module."
println(randomFunction())
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