sed to python: replace Q

For some reason I'm unable to grok Python's string.replace() function.
Just trying to parse a simple IP address, wrapped in square brackets,
from Postfix logs. In sed this is straightforward given:

line = "date process text [ip] more text"

  sed -e 's/^.*\[//' -e 's/].*$//'

yet the following Python code does nothing:

  line = line.replace('^.*\[', '', 1)
  line = line.replace('].*$', '')

Is there a decent description of string.replace() somewhere?

Raymond
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