new: check if existing pyproject.toml contains a poetry or build-syst…

…em section. If not prepend data on init.

poetry/console/commands/init.py

@@ -10,6 +10,8 @@
 from typing import Tuple
 from typing import Union
 
+import tomlkit
+
 from cleo import option
 from tomlkit import inline_table
 
@@ -22,7 +24,6 @@
 
 
 class InitCommand(Command):
-
     name = "init"
     description = (
         "Creates a basic <comment>pyproject.toml</> file in the current directory."
@@ -66,9 +67,26 @@ def handle(self):
         from poetry.utils.env import SystemEnv
         from poetry.core.vcs.git import GitConfig
 
+        original_toml = None
+
         if (Path.cwd() / "pyproject.toml").exists():
-            self.line("<error>A pyproject.toml file already exists.</error>")
-            return 1
+            with (Path.cwd() / "pyproject.toml").open() as toml_file:
+                original_toml = tomlkit.loads(toml_file.read())
+
+                try:
+                    if original_toml["tool"]["poetry"]:
+                        self.line(
+                            "<error>A pyproject.toml file with a poetry section already exists.</error>"
+                        )
+                        return 1
+                except KeyError:
+                    pass
+
+                if original_toml.get("build-system"):
+                    self.line(
+                        "<error>A pyproject.toml file with a defined build-system already exists.</error>"
+                    )
+                    return 1
 
         vcs_config = GitConfig()
 
@@ -202,6 +220,10 @@ def handle(self):
         with (Path.cwd() / "pyproject.toml").open("w", encoding="utf-8") as f:
             f.write(content)
 
+            if original_toml:
+                f.write("\n")
+                f.write(tomlkit.dumps(original_toml))
+
     def _determine_requirements(
         self, requires, allow_prereleases=False, source=None
     ):  # type: (List[str], bool) -> List[Dict[str, str]]