perf: implement lazy imports for heavy dependencies

Defer loading of heavy dependencies (networkx, matplotlib, click, pymysql)
until their associated features are accessed:

- dj.Diagram, dj.Di, dj.ERD -> loads diagram.py (networkx, matplotlib)
- dj.kill -> loads admin.py (pymysql via connection)
- dj.cli -> loads cli.py (click)

This reduces `import datajoint` time significantly, especially on macOS
where import overhead is higher. Core functionality (Schema, Table,
Connection, etc.) remains immediately available.

Closes #1220

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dimitri-yatsenko

src/datajoint/__init__.py

@@ -60,18 +60,18 @@
     "ValidationResult",
 ]
 
+# =============================================================================
+# Eager imports — core functionality needed immediately
+# =============================================================================
 from . import errors
 from . import migrate
-from .admin import kill
 from .codecs import (
     Codec,
     get_codec,
     list_codecs,
 )
 from .blob import MatCell, MatStruct
-from .cli import cli
 from .connection import Connection, conn
-from .diagram import Diagram
 from .errors import DataJointError
 from .expression import AndList, Not, Top, U
 from .hash import key_hash
@@ -83,5 +83,32 @@
 from .user_tables import Computed, Imported, Lookup, Manual, Part
 from .version import __version__
 
-ERD = Di = Diagram  # Aliases for Diagram
-schema = Schema  # Aliases for Schema
+schema = Schema  # Alias for Schema
+
+# =============================================================================
+# Lazy imports — heavy dependencies loaded on first access
+# =============================================================================
+# These modules import heavy dependencies (networkx, matplotlib, click, pymysql)
+# that slow down `import datajoint`. They are loaded on demand.
+
+_lazy_modules = {
+    # Diagram imports networkx and matplotlib
+    "Diagram": (".diagram", "Diagram"),
+    "Di": (".diagram", "Diagram"),
+    "ERD": (".diagram", "Diagram"),
+    # kill imports pymysql via connection
+    "kill": (".admin", "kill"),
+    # cli imports click
+    "cli": (".cli", "cli"),
+}
+
+
+def __getattr__(name: str):
+    """Lazy import for heavy dependencies."""
+    if name in _lazy_modules:
+        module_path, attr_name = _lazy_modules[name]
+        import importlib
+
+        module = importlib.import_module(module_path, __package__)
+        return getattr(module, attr_name)
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")

tests/unit/test_lazy_imports.py

@@ -0,0 +1,105 @@
+"""
+Tests for lazy import behavior.
+
+These tests verify that heavy dependencies (networkx, matplotlib, click)
+are not loaded until their associated features are accessed.
+"""
+
+import sys
+
+
+def test_lazy_diagram_import():
+    """Diagram module should not be loaded until dj.Diagram is accessed."""
+    # Remove datajoint from sys.modules to get fresh import
+    modules_to_remove = [key for key in sys.modules if key.startswith("datajoint")]
+    for mod in modules_to_remove:
+        del sys.modules[mod]
+
+    # Import datajoint
+    import datajoint as dj
+
+    # Diagram module should not be loaded yet
+    assert "datajoint.diagram" not in sys.modules, "diagram module loaded eagerly"
+
+    # Access Diagram - should trigger lazy load
+    Diagram = dj.Diagram
+    assert "datajoint.diagram" in sys.modules, "diagram module not loaded after access"
+    assert Diagram.__name__ == "Diagram"
+
+
+def test_lazy_admin_import():
+    """Admin module should not be loaded until dj.kill is accessed."""
+    # Remove datajoint from sys.modules to get fresh import
+    modules_to_remove = [key for key in sys.modules if key.startswith("datajoint")]
+    for mod in modules_to_remove:
+        del sys.modules[mod]
+
+    # Import datajoint
+    import datajoint as dj
+
+    # Admin module should not be loaded yet
+    assert "datajoint.admin" not in sys.modules, "admin module loaded eagerly"
+
+    # Access kill - should trigger lazy load
+    kill = dj.kill
+    assert "datajoint.admin" in sys.modules, "admin module not loaded after access"
+    assert callable(kill)
+
+
+def test_lazy_cli_import():
+    """CLI module should not be loaded until dj.cli is accessed."""
+    # Remove datajoint from sys.modules to get fresh import
+    modules_to_remove = [key for key in sys.modules if key.startswith("datajoint")]
+    for mod in modules_to_remove:
+        del sys.modules[mod]
+
+    # Import datajoint
+    import datajoint as dj
+
+    # CLI module should not be loaded yet
+    assert "datajoint.cli" not in sys.modules, "cli module loaded eagerly"
+
+    # Access cli - should trigger lazy load
+    _ = dj.cli
+    assert "datajoint.cli" in sys.modules, "cli module not loaded after access"
+
+
+def test_diagram_aliases():
+    """Di and ERD should be aliases for Diagram."""
+    # Remove datajoint from sys.modules to get fresh import
+    modules_to_remove = [key for key in sys.modules if key.startswith("datajoint")]
+    for mod in modules_to_remove:
+        del sys.modules[mod]
+
+    import datajoint as dj
+
+    # All aliases should resolve to the same class
+    assert dj.Diagram is dj.Di
+    assert dj.Diagram is dj.ERD
+
+
+def test_core_imports_available():
+    """Core functionality should be available immediately after import."""
+    # Remove datajoint from sys.modules to get fresh import
+    modules_to_remove = [key for key in sys.modules if key.startswith("datajoint")]
+    for mod in modules_to_remove:
+        del sys.modules[mod]
+
+    import datajoint as dj
+
+    # Core classes should be available without triggering lazy loads
+    assert hasattr(dj, "Schema")
+    assert hasattr(dj, "Table")
+    assert hasattr(dj, "Manual")
+    assert hasattr(dj, "Lookup")
+    assert hasattr(dj, "Computed")
+    assert hasattr(dj, "Imported")
+    assert hasattr(dj, "Part")
+    assert hasattr(dj, "Connection")
+    assert hasattr(dj, "config")
+    assert hasattr(dj, "errors")
+
+    # Heavy modules should still not be loaded
+    assert "datajoint.diagram" not in sys.modules
+    assert "datajoint.admin" not in sys.modules
+    assert "datajoint.cli" not in sys.modules