A tool (and pre-commit hook) to automatically add trailing commas to calls and literals.

pip install add-trailing-comma

See pre-commit for instructions

Sample .pre-commit-config.yaml:

-   repo: https://github.com/asottile/add-trailing-comma
    rev: v2.1.0
    hooks:
    -   id: add-trailing-comma

# Sample of *ideal* syntax
function_call(
    argument,
    5 ** 5,
    kwarg=foo,
)

• the initial paren is at the end of the line
• each argument is indented one level further than the function name
• the last parameter (unless the call contains an unpacking (*args / **kwargs)) has a trailing comma

This has the following benefits:

• arbitrary indentation is avoided:

  # I hear you like 15 space indents
  # oh your function name changed? guess you get to reindent :)
  very_long_call(arg,
                 arg,
                 arg)

• adding / removing a parameter preserves git blame and is a minimal diff:

   # with no trailing commas
   x(
  -    arg
  +    arg,
  +    arg2
   )

   # with trailing commas
   x(
       arg,
  +    arg2,
   )

 x(
     arg,
-    arg
+    arg,
 )

If --py35-plus is passed, add-trailing-comma will also perform the following change:

 x(
-    *args
+    *args,
 )
 y(
-    **kwargs
+    **kwargs,
 )

Note that this would cause a SyntaxError in earlier python versions.

 x = [
-    1, 2, 3
+    1, 2, 3,
 ]

 def func(
         arg1,
-        arg2
+        arg2,
 ):

 async def func(
         arg1,
-        arg2
+        arg2,
 ):

If --py36-plus is passed, add-trailing-comma will also perform the following change:

 def f(
-    *args
+    *args,
 ): pass


 def g(
-    **kwargs
+    **kwargs,
 ): pass


 def h(
-    *, kw=1
+    *, kw=1,
 ): pass

Note that this would cause a SyntaxError in earlier python versions.

 from os import (
     path,
-    makedirs
+    makedirs,
 )

 class C(
     Base1,
-    Base2
+    Base2,
 ):
     pass

 x(
     arg1,
-    arg2)
+    arg2,
+)

-function_name(arg1,
-              arg2)
+function_name(
+    arg1,
+    arg2,
+)

 x = [
     1,
     2,
     3,
-    ]
+]

yes yes, I realize the tool is called add-trailing-comma 😆

-[1, 2, 3,]
-[1, 2, 3, ]
+[1, 2, 3]
+[1, 2, 3]