rstrip removes the trailing `e`s. 

i = "external_eeeeeeeee_object"
i = i.rstrip('_object')
print(i) 
"""
It should have printed `external_eeeeeeeee` but it prints only `external_`. 
"""

It happens only when I user rstrip("_object") not for other strings. 

# =======================================================
# it works fine in the below case. 


i = "external_eeeeeeeee_trail"
i = i.rstrip('_trail')
print(i) 
"""
It should have prints `external_eeeeeeeee`
"""

See https://docs.python.org/3/library/stdtypes.html#str.rstrip

The `{l,r,}strip` methods remove all characters contained in the passed-in string; `"aabbccddeeffgg".rstrip("gfe") == "aabbccdd"`

For 3.9+, you could do exactly what you want with .removesuffix (/.removeprefix) methods;
>>> test = "external_eeeeeeeee_object"
>>> test.removesuffix("_object")
'external_eeeeeeeee'