Close #19379: Lazily import linecache in the warnings module, to make startup…

Close #19379: Lazily import linecache in the warnings module, to make startup with warnings faster until a warning gets printed.

Lib/warnings.py

 """Python part of the warnings subsystem."""
 
-# Note: function level imports should *not* be used
-# in this module as it may cause import lock deadlock.
-# See bug 683658.
-import linecache
 import sys
 
 __all__ = ["warn", "showwarning", "formatwarning", "filterwarnings",
@@ -21,6 +17,7 @@ def showwarning(message, category, filename, lineno, file=None, line=None):
 
 def formatwarning(message, category, filename, lineno, line=None):
     """Function to format a warning the standard way."""
+    import linecache
     s =  "%s:%s: %s: %s\n" % (filename, lineno, category.__name__, message)
     line = linecache.getline(filename, lineno) if line is None else line
     if line:
@@ -233,6 +230,7 @@ def warn_explicit(message, category, filename, lineno,
 
     # Prime the linecache for formatting, in case the
     # "file" is actually in a zipfile or something.
+    import linecache
     linecache.getlines(filename, module_globals)
 
     if action == "error":

Misc/NEWS

@@ -21,6 +21,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #19379: Lazily import linecache in the warnings module, to make
+  startup with warnings faster until a warning gets printed.
+
 - Issue #19327: Fixed the working of regular expressions with too big charset.
 
 - Issue #17400: New 'is_global' attribute for ipaddress to tell if an address