How can I pass an empty string into triple quoted f-string in Python?

Question

Here is a function using a triple quoted f-string with lots of sub-elements:

def pass_empty_string(param):
    from lxml import etree
    xml = etree.XML(f'''
    <root>
        <child>text</child>
        <child>{param}</child>
        ...
    </root>''')
    return xml

Is it possible to get an empty </child> element when param is getting None or '' value?

have you tried this? If you pass `''` then a child is empty but passing `None` has `None`; the other elements are present if you just want an empty child element see answer below — depperm, Feb 09 '18 at 15:52

erip · Accepted Answer · 2018-02-09T16:00:44.840

5

You can accomplish this with or:

f"<child>{param or ''}</child>"

Anything in the braces is evaluated as an expression, so...

>>> param = None
>>> f"<child>{param or ''}</child>"
'<child></child>'
>>> param = ''
>>> f"<child>{param or ''}</child>"
'<child></child>'
>>> param = "some valid child"
>>> f"<child>{param or ''}</child>"
'<child>some valid child</child>'

Both '' and None are falsy values, so it will fall back to the RHS of the or, which will be just an empty string.

edited Feb 09 '18 at 16:00

answered Feb 09 '18 at 15:57

erip

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Didn't know you could do this! – Farhan.K Feb 09 '18 at 16:00
2

@Farhan.K Anything in the `{}` is evaluated as an expression -- this is the power of f-strings. – erip Feb 09 '18 at 16:01
Very cool. You can even execute ternary conditions like `f"{param1 if param1 else param2}"` within the braces. Learned something new, thanks! – r.ook Feb 09 '18 at 16:02
2

@Idlehands You could even have another `f`-string within the `{...}` ;-) – tobias_k Feb 09 '18 at 16:04

score 1 · Answer 2 · edited Feb 11 '18 at 14:11

1

Couldn't you just use if statements?

def pass_empty_string(param):
    from lxml import etree
    if param is None or param == '':
        return etree.XML(f'<child></child>')
    xml = etree.XML(f'''
    <root>
        <child>text</child>
        <child>{param}</child>
        ...
    </root>''')
    return xml

edited Feb 11 '18 at 14:11

erip

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answered Feb 09 '18 at 15:52

SuperStew

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score 1 · Answer 3 · edited Feb 10 '18 at 19:56

1

A simple if statement will do:

def pass_empty_string(param):
    if not param:
        param = ''
    xml = etree.XML(f'''<root>
            <child>text</child>
            <child>{param}</child>
        </root>''')
    return xml

xml looks like this:

<root>
    <child>text</child>
    <child></child>
</root>

edited Feb 10 '18 at 19:56

erip

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answered Feb 09 '18 at 15:56

Farhan.K

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How can I pass an empty string into triple quoted f-string in Python?

3 Answers3