Trying to use Append To File raises AttributeError exception - robotframework

Question

I am trying to append data stored in a variable to a file. The file was successfully created and the data is stored in the variable; however, I continue to get the AttiributeError:
AttributeError: 'NoneType' object has no attribute 'replace'
here are my results
KEYWORD ${FileA} = OperatingSystem . Create File metrics/Live_RTT.txt, ${MT_Com_RTT_LB_Live_Topics}
00:00:05.233 KEYWORD Selenium2Library . Click Element ${BLANK_CANVAS}
00:00:05.129 KEYWORD Selenium2Library . Click Element //div[#title='${device1}']
00:00:05.063 KEYWORD Selenium2Library . Click Element //div[#title='${device2}']
00:00:00.045 KEYWORD ${MT_Com_RTT_LB_Live_Topics} = Selenium2Library . Get Text xpath=${REAL_TIME_TRENDING_TOPICS}
00:00:00.001 KEYWORD BuiltIn . Log ${MT_Com_RTT_LB_Live_Topics}
Documentation:
Logs the given message with the given level.
Start / End / Elapsed: 20181028 21:37:34.971 / 20181028 21:37:34.972 / 00:00:00.001
21:37:34.971 INFO Available Topics
Active Energy Delivered + Received
Active Energy Into the Load
Active Energy Out of the Load
Active Power
Active Power A
00:00:00.001 KEYWORD OperatingSystem . Append To File ${FileA}, ${MT_Com_RTT_LB_Live_Topics}
Documentation:
Appends the given content to the specified file.
Start / End / Elapsed: 20181028 21:37:34.972 / 20181028 21:37:34.973 / 00:00:00.001
21:37:34.973 FAIL AttributeError: 'NoneType' object has no attribute 'replace'

Answer 1

Looking at the library's source, the exception most probably is raised in the normalize_path() method, which does this string replacement:
path = os.path.normpath(os.path.expanduser(path.replace('/', os.sep)))
, where path is the filename - the first argument to the Append To File kw call. The exception says its value is None (e.g. no value set) while it should be some string.
Looking at the log, you're setting this value to the return value of Create File - but that keyword does not return one at all, it simply creates the file you specified in its first argument.
So to resolve - just set the value of ${FileA} yourself, pass it to the two keywords, and do not reassign in the Create File call.