[robotframework]为什么给列表创建一个空元素可以视为True，但不适用于空变量

Question

Create a empty element to a list, and return True value with "Should Be True" key word. but not suiteable for create a empty variable.Why is that? code:

${list1}    Create List    ${EMPTY}
${list2}    Set Variable    ${EMPTY}
Should Be True    ${list1}    Create empty list
Should Be True    ${list2}    Set empty variable

Log:

20211225 14:24:58.913: INFO: ${list1} = ['']

20211225 14:24:58.916: TRACE: Arguments: [ '' ]

20211225 14:24:58.916: TRACE: Return: ''

20211225 14:24:58.917: INFO: ${list2} =

20211225 14:24:58.918: TRACE: Arguments: [ [''] | 'Create empty list' ]

20211225 14:24:58.918: TRACE: Return: None

20211225 14:24:58.919: TRACE: Arguments: [ '' | 'Set empty variable' ]

20211225 14:24:58.920: FAIL: Evaluating expression '' failed: ValueError: Expression cannot be empty.

Answer 1

It is a bit counter-intuitive because ${list1} Create List ${EMPTY} is not creating an empty list, but a list with an empty element inside, eg [''] , while ${list2} Set Variable ${EMPTY} will create an empty variable, eg '' . This is why Should Be True passes on the list (list is not empty) but fails on the variable (variable is empty).

Below code exemplifies these cases:

    # empty_list == []
    ${empty_list} =     Create List
    log to console      ${empty_list}
    should be empty     ${empty_list}

    # empty_list_1 == ['']
    ${empty_list_1} =    Create List    ${EMPTY}
    log to console       ${empty_list_1}
    should not be empty  ${empty_list_1}

    # empty_var == '' (empty string with length 0)
    ${empty_var} =       Set Variable
    log to console       ${empty_var}
    should be empty      ${empty_var}

    # empty_var_1 == '' (empty string with length 0)
    ${empty_var_1} =     Set Variable    ${EMPTY}
    log to console       ${empty_var_1}
    should be empty      ${empty_var_1}