Extract get_block_string_indentation function

Replicates graphql/graphql-js@280edb6

README.md

@@ -13,7 +13,7 @@ a query language for APIs created by Facebook.
 [![Code Style](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/ambv/black)
 
 The current version 1.0.2 of GraphQL-core-next is up-to-date with GraphQL.js version
-14.2.1. All parts of the API are covered by an extensive test suite of currently 1748
+14.2.1. All parts of the API are covered by an extensive test suite of currently 1753
 unit tests.
 
 

graphql/language/block_string.py

@@ -1,4 +1,10 @@
-__all__ = ["dedent_block_string_value", "print_block_string"]
+from typing import List
+
+__all__ = [
+    "dedent_block_string_value",
+    "print_block_string",
+    "get_block_string_indentation",
+]
 
 
 def dedent_block_string_value(raw_string: str) -> str:
@@ -9,28 +15,42 @@ def dedent_block_string_value(raw_string: str) -> str:
 
     This implements the GraphQL spec's BlockStringValue() static algorithm.
     """
+    # Expand a block string's raw value into independent lines.
     lines = raw_string.splitlines()
 
-    common_indent = None
-    for line in lines[1:]:
-        indent = leading_whitespace(line)
-        if indent < len(line) and (common_indent is None or indent < common_indent):
-            common_indent = indent
-        if common_indent == 0:
-            break
+    # Remove common indentation from all lines but first.
+    common_indent = get_block_string_indentation(lines)
 
     if common_indent:
         lines[1:] = [line[common_indent:] for line in lines[1:]]
 
+    # Remove leading and trailing blank lines.
     while lines and not lines[0].strip():
         lines = lines[1:]
 
     while lines and not lines[-1].strip():
         lines = lines[:-1]
 
+    # Return a string of the lines joined with U+000A.
     return "\n".join(lines)
 
 
+def get_block_string_indentation(lines: List[str]) -> int:
+    common_indent = None
+
+    for line in lines[1:]:
+        indent = leading_whitespace(line)
+        if indent == len(line):
+            continue  # skip empty lines
+
+        if common_indent is None or indent < common_indent:
+            common_indent = indent
+            if common_indent == 0:
+                break
+
+    return 0 if common_indent is None else common_indent
+
+
 def leading_whitespace(s):
     i = 0
     n = len(s)

tests/language/test_block_string.py

@@ -1,4 +1,8 @@
-from graphql.language.block_string import dedent_block_string_value, print_block_string
+from graphql.language.block_string import (
+    dedent_block_string_value,
+    print_block_string,
+    get_block_string_indentation,
+)
 
 
 def join_lines(*args):
@@ -69,6 +73,32 @@ def does_not_alter_trailing_spaces():
         )
 
 
+def describe_get_block_string_indentation():
+    def returns_zero_for_an_empty_list():
+        assert get_block_string_indentation([]) == 0
+
+    def do_not_take_first_line_into_account():
+        assert get_block_string_indentation(["  a"]) == 0
+        assert get_block_string_indentation([" a", "  b"]) == 2
+
+    def returns_minimal_indentation_length():
+        assert get_block_string_indentation(["", " a", "  b"]) == 1
+        assert get_block_string_indentation(["", "  a", " b"]) == 1
+        assert get_block_string_indentation(["", "  a", " b", "c"]) == 0
+
+    def count_both_tab_and_space_as_single_character():
+        assert get_block_string_indentation(["", "\ta", "          b"]) == 1
+        assert get_block_string_indentation(["", "\t a", "          b"]) == 2
+        assert get_block_string_indentation(["", " \t a", "          b"]) == 3
+
+    def do_not_take_empty_lines_into_account():
+        assert get_block_string_indentation((["a", "\t"])) == 0
+        assert get_block_string_indentation((["a", " "])) == 0
+        assert get_block_string_indentation((["a", " ", "  b"])) == 2
+        assert get_block_string_indentation((["a", " ", "  b"])) == 2
+        assert get_block_string_indentation((["a", "", " b"])) == 1
+
+
 def describe_print_block_string():
     def by_default_print_block_strings_as_single_line():
         s = "one liner"