parsing: make imports lazy (#5225)

* parsing: make imports lazy

* Locally declare default params file

dvc/parsing/__init__.py

@@ -5,7 +5,6 @@ from typing import TYPE_CHECKING, Any, Dict, NamedTuple, Union
 
 from funcy import cached_property, collecting, first, isa, join, reraise
 
-from dvc.dependency.param import ParamsDependency
 from dvc.exceptions import DvcException
 from dvc.parsing.interpolate import ParseError
 from dvc.path_info import PathInfo
@@ -34,11 +33,11 @@ logger = logging.getLogger(__name__)
 STAGES_KWD = "stages"
 VARS_KWD = "vars"
 WDIR_KWD = "wdir"
-DEFAULT_PARAMS_FILE = ParamsDependency.DEFAULT_PARAMS_FILE
 PARAMS_KWD = "params"
 FOREACH_KWD = "foreach"
 DO_KWD = "do"
 
+DEFAULT_PARAMS_FILE = "params.yaml"
 DEFAULT_SENTINEL = object()
 
 JOIN = "@"

dvc/parsing/context.py

@@ -21,8 +21,6 @@ from dvc.parsing.interpolate import (
 )
 from dvc.path_info import PathInfo
 from dvc.utils import relpath
-from dvc.utils.humanize import join
-from dvc.utils.serialize import LOADERS
 
 logger = logging.getLogger(__name__)
 SeqOrMap = Union[Sequence, Mapping]
@@ -34,6 +32,8 @@ class ContextError(DvcException):
 
 class ReservedKeyError(ContextError):
     def __init__(self, keys, path=None):
+        from dvc.utils.humanize import join
+
         self.keys = keys
         self.path = path
 
@@ -347,6 +347,8 @@ class Context(CtxDict):
 
     @classmethod
     def load_from(cls, tree, path: PathInfo, select_keys=None) -> "Context":
+        from dvc.utils.serialize import LOADERS
+
         file = relpath(path)
         if not tree.exists(path):
             raise ParamsLoadError(f"'{file}' does not exist")

dvc/parsing/interpolate.py

@@ -6,7 +6,6 @@ from functools import singledispatch
 from funcy import memoize, rpartial
 
 from dvc.exceptions import DvcException
-from dvc.utils import colorize
 
 if typing.TYPE_CHECKING:
     from typing import List, Match
@@ -84,6 +83,8 @@ def _(obj: bool):
 def _format_exc_msg(exc: "ParseException"):
     from pyparsing import ParseException
 
+    from dvc.utils import colorize
+
     exc.loc += 2  # 2 because we append `${` at the start of expr below
 
     expr = exc.pstr