[925] Improve multiline dictionary and list indentation for sole function parameter

CHANGES.md

@@ -13,6 +13,8 @@
 ### Preview style
 
 - Fix merging implicit multiline strings that have inline comments (#3956)
+- Multiline dictionaries and lists that are the sole argument to a function are now
+  indented less (#3964)
 
 ### Configuration
 

docs/the_black_code_style/future_style.md

@@ -113,6 +113,32 @@ my_dict = {
 }
 ```
 
+### Improved multiline dictionary and list indentation for sole function parameter
+
+For better readability and less verticality, _Black_ now pairs parantheses ("(", ")")
+with braces ("{", "}") and square brackets ("[", "]") on the same line for single
+parameter function calls. For example:
+
+```python
+foo(
+    [
+        1,
+        2,
+        3,
+    ]
+)
+```
+
+will be changed to:
+
+```python
+foo([
+    1,
+    2,
+    3,
+])
+```
+
 ### Improved multiline string handling
 
 _Black_ is smarter when formatting multiline strings, especially in function arguments,

src/black/linegen.py

@@ -815,6 +815,20 @@ def _first_right_hand_split(
     tail_leaves.reverse()
     body_leaves.reverse()
     head_leaves.reverse()
+
+    if Preview.hug_parens_with_braces_and_square_brackets in line.mode:
+        if (
+            tail_leaves[0].type == token.RPAR
+            and tail_leaves[0].value
+            and head_leaves[-1].type == token.LPAR
+            and head_leaves[-1].value
+            and body_leaves[0].type in [token.LBRACE, token.LSQB]
+            and body_leaves[-1].type in [token.RBRACE, token.RSQB]
+        ):
+            head_leaves = head_leaves + body_leaves[:1]
+            tail_leaves = body_leaves[-1:] + tail_leaves
+            body_leaves = body_leaves[1:-1]
+
     head = bracket_split_build_line(
         head_leaves, line, opening_bracket, component=_BracketSplitComponent.head
     )

src/black/mode.py

@@ -190,6 +190,7 @@ class Preview(Enum):
     module_docstring_newlines = auto()
     accept_raw_docstrings = auto()
     fix_power_op_line_length = auto()
+    hug_parens_with_braces_and_square_brackets = auto()
 
 
 class Deprecated(UserWarning):

tests/data/cases/preview_hug_parens_with_braces_and_square_brackets.py

@@ -0,0 +1,258 @@
+# flags: --preview
+def foo_brackets(request):
+    return JsonResponse(
+        {
+            "var_1": foo,
+            "var_2": bar,
+        }
+    )
+
+def foo_square_brackets(request):
+    return JsonResponse(
+        [
+            "var_1",
+            "var_2",
+        ]
+    )
+
+func({"a": 37, "b": 42, "c": 927, "aaaaaaaaaaaaaaaaaaaaaaaaa": 11111111111111111111111111111111111111111})
+
+func(["random_string_number_one","random_string_number_two","random_string_number_three","random_string_number_four"])
+
+func(
+    {
+        # expand me
+        'a':37,
+        'b':42,
+        'c':927
+    }
+)
+
+func(
+    [
+        'a',
+        'b',
+        'c',
+    ]
+)
+
+func(  # a
+    [  # b
+        "c",  # c
+        "d",  # d
+        "e",  # e
+    ]  # f
+)  # g
+
+func(  # a
+    {  # b
+        "c": 1,  # c
+        "d": 2,  # d
+        "e": 3,  # e
+    }  # f
+)  # g
+
+func(
+    # preserve me
+    [
+        "c",
+        "d",
+        "e",
+    ]
+)
+
+func(
+    [  # preserve me but hug brackets
+        "c",
+        "d",
+        "e",
+    ]
+)
+
+func(
+    [
+        # preserve me but hug brackets
+        "c",
+        "d",
+        "e",
+    ]
+)
+
+func(
+    [
+        "c",
+        # preserve me but hug brackets
+        "d",
+        "e",
+    ]
+)
+
+func(
+    [
+        "c",
+        "d",
+        "e",
+        # preserve me but hug brackets
+    ]
+)
+
+func(
+    [
+        "c",
+        "d",
+        "e",
+    ]  # preserve me but hug brackets
+)
+
+func(
+    [
+        "c",
+        "d",
+        "e",
+    ]
+    # preserve me
+)
+
+func([x for x in "short line"])
+func([x for x in "long line long line long line long line long line long line long line"])
+func([x for x in [x for x in "long line long line long line long line long line long line long line"]])
+
+func({"short line"})
+func({"long line", "long long line", "long long long line", "long long long long line", "long long long long long line"})
+func({{"long line", "long long line", "long long long line", "long long long long line", "long long long long long line"}})
+
+
+# output
+def foo_brackets(request):
+    return JsonResponse({
+        "var_1": foo,
+        "var_2": bar,
+    })
+
+
+def foo_square_brackets(request):
+    return JsonResponse([
+        "var_1",
+        "var_2",
+    ])
+
+
+func({
+    "a": 37,
+    "b": 42,
+    "c": 927,
+    "aaaaaaaaaaaaaaaaaaaaaaaaa": 11111111111111111111111111111111111111111,
+})
+
+func([
+    "random_string_number_one",
+    "random_string_number_two",
+    "random_string_number_three",
+    "random_string_number_four",
+])
+
+func({
+    # expand me
+    "a": 37,
+    "b": 42,
+    "c": 927,
+})
+
+func([
+    "a",
+    "b",
+    "c",
+])
+
+func([  # a  # b
+    "c",  # c
+    "d",  # d
+    "e",  # e
+])  # f  # g
+
+func({  # a  # b
+    "c": 1,  # c
+    "d": 2,  # d
+    "e": 3,  # e
+})  # f  # g
+
+func(
+    # preserve me
+    [
+        "c",
+        "d",
+        "e",
+    ]
+)
+
+func([  # preserve me but hug brackets
+    "c",
+    "d",
+    "e",
+])
+
+func([
+    # preserve me but hug brackets
+    "c",
+    "d",
+    "e",
+])
+
+func([
+    "c",
+    # preserve me but hug brackets
+    "d",
+    "e",
+])
+
+func([
+    "c",
+    "d",
+    "e",
+    # preserve me but hug brackets
+])
+
+func([
+    "c",
+    "d",
+    "e",
+])  # preserve me but hug brackets
+
+func(
+    [
+        "c",
+        "d",
+        "e",
+    ]
+    # preserve me
+)
+
+func([x for x in "short line"])
+func([
+    x for x in "long line long line long line long line long line long line long line"
+])
+func([
+    x
+    for x in [
+        x
+        for x in "long line long line long line long line long line long line long line"
+    ]
+])
+
+func({"short line"})
+func({
+    "long line",
+    "long long line",
+    "long long long line",
+    "long long long long line",
+    "long long long long long line",
+})
+func({
+    {
+        "long line",
+        "long long line",
+        "long long long line",
+        "long long long long line",
+        "long long long long long line",
+    }
+})

tests/data/cases/preview_long_strings__regression.py

@@ -962,19 +962,17 @@ def who(self):
         )
 
 
-xxxxxxx_xxxxxx_xxxxxxx = xxx(
-    [
-        xxxxxxxxxxxx(
-            xxxxxx_xxxxxxx=(
-                '((x.aaaaaaaaa = "xxxxxx.xxxxxxxxxxxxxxxxxxxxx") || (x.xxxxxxxxx ='
-                ' "xxxxxxxxxxxx")) && '
-                # xxxxx xxxxxxxxxxxx xxxx xxx (xxxxxxxxxxxxxxxx) xx x xxxxxxxxx xx xxxxxx.
-                "(x.bbbbbbbbbbbb.xxx != "
-                '"xxx:xxx:xxx::cccccccccccc:xxxxxxx-xxxx/xxxxxxxxxxx/xxxxxxxxxxxxxxxxx") && '
-            )
+xxxxxxx_xxxxxx_xxxxxxx = xxx([
+    xxxxxxxxxxxx(
+        xxxxxx_xxxxxxx=(
+            '((x.aaaaaaaaa = "xxxxxx.xxxxxxxxxxxxxxxxxxxxx") || (x.xxxxxxxxx ='
+            ' "xxxxxxxxxxxx")) && '
+            # xxxxx xxxxxxxxxxxx xxxx xxx (xxxxxxxxxxxxxxxx) xx x xxxxxxxxx xx xxxxxx.
+            "(x.bbbbbbbbbbbb.xxx != "
+            '"xxx:xxx:xxx::cccccccccccc:xxxxxxx-xxxx/xxxxxxxxxxx/xxxxxxxxxxxxxxxxx") && '
         )
-    ]
-)
+    )
+])
 
 if __name__ == "__main__":
     for i in range(4, 8):