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Reference article: A Concise Guide to XPATH

XPath variables use the $somevariable syntax, which is the $ symbol plus the variable name, and then the parameter variable value is passed when the xpath method is called.

>>> # `$val` used in the expression, a `val` argument needs to be passed
>>> response.xpath('//p[@id=$val]/a/text()', val='images').extract_first()
u'Name: My image 1 '

response.xpath('//p[@id={}]'.format(val)).extract_first()

I understand that the parameter of xpath is also a string, please try it.

Scrapy Documentation

This is a python statement. Why not use string splicing to splice this expression together?
For example

response.xpath('//p[@id=' + val + ']').extract_first()