Begin removing .format in favor of f-strings

docs/guide.md

@@ -30,7 +30,7 @@ the program to the command line.
 import fire
 
 def hello(name):
-  return 'Hello {name}!'.format(name=name)
+  return f'Hello {name}!'
 
 if __name__ == '__main__':
   fire.Fire()
@@ -52,7 +52,7 @@ command line.
 import fire
 
 def hello(name):
-  return 'Hello {name}!'.format(name=name)
+  return f'Hello {name}!'
 
 if __name__ == '__main__':
   fire.Fire(hello)
@@ -76,7 +76,7 @@ We can alternatively write this program like this:
 import fire
 
 def hello(name):
-  return 'Hello {name}!'.format(name=name)
+  return f'Hello {name}!'
 
 def main():
   fire.Fire(hello)
@@ -93,7 +93,7 @@ then simply this:
 import fire
 
 def hello(name):
-  return 'Hello {name}!'.format(name=name)
+  return f'Hello {name}!'
 
 def main():
   fire.Fire(hello)
@@ -105,7 +105,7 @@ If you have a file `example.py` that doesn't even import fire:
 
 ```python
 def hello(name):
-  return 'Hello {name}!'.format(name=name)
+  return f'Hello {name}!'
 ```
 
 Then you can use it with Fire like this:

fire/core.py

@@ -139,15 +139,15 @@ def Fire(component=None, command=None, name=None, serialize=None):
     _DisplayError(component_trace)
     raise FireExit(2, component_trace)
   if component_trace.show_trace and component_trace.show_help:
-    output = ['Fire trace:\n{trace}\n'.format(trace=component_trace)]
+    output = [f'Fire trace:\n{component_trace}\n']
     result = component_trace.GetResult()
     help_text = helptext.HelpText(
         result, trace=component_trace, verbose=component_trace.verbose)
     output.append(help_text)
     Display(output, out=sys.stderr)
     raise FireExit(0, component_trace)
   if component_trace.show_trace:
-    output = ['Fire trace:\n{trace}'.format(trace=component_trace)]
+    output = [f'Fire trace:\n{component_trace}']
     Display(output, out=sys.stderr)
     raise FireExit(0, component_trace)
   if component_trace.show_help:
@@ -231,9 +231,9 @@ def _IsHelpShortcut(component_trace, remaining_args):
 
   if show_help:
     component_trace.show_help = True
-    command = '{cmd} -- --help'.format(cmd=component_trace.GetCommand())
-    print('INFO: Showing help with the command {cmd}.\n'.format(
-        cmd=shlex.quote(command)), file=sys.stderr)
+    command = f'{component_trace.GetCommand()} -- --help'
+    print(f'INFO: Showing help with the command {shlex.quote(command)}.\n',
+          file=sys.stderr)
   return show_help
 
 
@@ -287,9 +287,9 @@ def _DisplayError(component_trace):
       show_help = True
 
   if show_help:
-    command = '{cmd} -- --help'.format(cmd=component_trace.GetCommand())
-    print('INFO: Showing help with the command {cmd}.\n'.format(
-        cmd=shlex.quote(command)), file=sys.stderr)
+    command = f'{component_trace.GetCommand()} -- --help'
+    print(f'INFO: Showing help with the command {shlex.quote(command)}.\n',
+          file=sys.stderr)
     help_text = helptext.HelpText(result, trace=component_trace,
                                   verbose=component_trace.verbose)
     output.append(help_text)
@@ -327,14 +327,13 @@ def _DictAsString(result, verbose=False):
     return '{}'
 
   longest_key = max(len(str(key)) for key in result_visible.keys())
-  format_string = '{{key:{padding}s}} {{value}}'.format(padding=longest_key + 1)
+  format_string = f'{{key:{longest_key + 1}s}} {{value}}'
 
   lines = []
   for key, value in result.items():
     if completion.MemberVisible(result, key, value, class_attrs=class_attrs,
                                 verbose=verbose):
-      line = format_string.format(key=str(key) + ':',
-                                  value=_OneLineResult(value))
+      line = format_string.format(key=f'{key}:', value=_OneLineResult(value))
       lines.append(line)
   return '\n'.join(lines)
 
@@ -348,10 +347,10 @@ def _OneLineResult(result):
   # TODO(dbieber): Show a small amount of usage information about the function
   # or module if it fits cleanly on the line.
   if inspect.isfunction(result):
-    return '<function {name}>'.format(name=result.__name__)
+    return f'<function {result.__name__}>'
 
   if inspect.ismodule(result):
-    return '<module {name}>'.format(name=result.__name__)
+    return f'<module {result.__name__}>'
 
   try:
     # Don't force conversion to ascii.
@@ -890,9 +889,10 @@ def _ParseKeywordArgs(args, fn_spec):
         if len(matching_fn_args) == 1:
           keyword = matching_fn_args[0]
         elif len(matching_fn_args) > 1:
-          raise FireError("The argument '{}' is ambiguous as it could "
-                          "refer to any of the following arguments: {}".format(
-                              argument, matching_fn_args))
+          raise FireError(
+              f"The argument '{argument}' is ambiguous as it could "
+              f"refer to any of the following arguments: {matching_fn_args}"
+          )
 
       # Determine the value.
       if not keyword: