code: def diff(
      self,
      a_rev: str = "HEAD",
      b_rev: Optional[str] = None,
      targets: Optional[List[str]] = None,
      recursive: bool = False,
  ):
      """
      By default, it compares the workspace with the last commit's fs.
  
      This implementation differs from `git diff` since DVC doesn't have
      the concept of `index`, but it keeps the same interface, thus,
      `dvc diff` would be the same as `dvc diff HEAD`.
      """
      if self.scm.no_commits:
          return {}
  
      indexes = {}
      missing_targets = defaultdict(set)
      with_missing = False
      if not b_rev:
          b_rev = "workspace"
          with_missing = True
  
      data_keys = set()
      for rev in self.brancher(revs=[a_rev, b_rev]):
          if rev == "workspace" and b_rev != "workspace":
              # brancher always returns workspace, but we only need to compute
              # workspace paths/checksums if b_rev was None
              continue
  
          def onerror(target, _exc):
              missing_targets[rev].add(target)  # noqa: B023
  
          view = self.index.targets_view(
              targets,
              onerror=onerror,
              recursive=recursive,
          )
  
          data_keys.update(view.data_keys.get("repo", set()))
  
          if rev == "workspace":
              from .index import build_data_index
  
              with ui.status("Building workspace index"):
                  data = build_data_index(
                      view,
                      self.root_dir,
                      self.fs,
                      compute_hash=True,
                  )
          else:
              data = view.data["repo"]
  
          assert rev not in indexes
          indexes[rev] = data
  
      if targets:
          old_missing = missing_targets.get(a_rev, set())
          new_missing = missing_targets.get(b_rev, set())
  
          # check for overlapping missing targets between a_rev and b_rev
          for target in old_missing & new_missing:
              raise FileNotFoundError(
                  errno.ENOENT, os.strerror(errno.ENOENT), target
              )
  
      if len(indexes.keys()) == 1:
          # both a_rev and b_rev point to the same sha, nothing to compare
          old = None
          new = None
      else:
          old = indexes[a_rev]
          new = indexes[b_rev]
  
      with ui.status("Calculating diff"):
          return _diff(old, new, data_keys, with_missing=with_missing)
  
file_path: dvc/dvc/repo/diff.py
imports: import errno
  import os
  from collections import defaultdict
  from typing import Dict, List, Optional
  from dvc.log import logger
  from dvc.repo import locked
  from dvc.ui import ui
label_desc: Use image node_id 4 for calling a global function with example usage: diff(a_rev, b_rev, targets, recursive) and returns: dict, _diff