python lxml xpath：如何使此谓词正常工作

Question

Good Morning,

Recently I picked up python and web scraping as a hobby ...

I'm trying to get my head around an issue with python lxml and xpath predicates but alas - apparently there's nothing similar on stackoverflow. So I managed to reproduce in below code, hoping someone sees what I don't ...

Is there somebody who can explain why the Result3 is an empty list? I was expecting Result3 to be the same as Result1.

How can I achieve that Result3 = Result1 ?

Versions: Python 3.7.3, lxml 4.4.0 (installed using pip, not Christoph Gohlke's binary) on an AMD windows machine.

Thanks in advance!

Stef

import lxml.html

simple_record  = """<a href="some_map/some_file.png">dododo</a>"""
tree           = lxml.html.fromstring(simple_record)

simple_xpath   = "@href"
found_field    = tree.xpath(simple_xpath)
print("Result1 = {}".format(found_field))

simple_xpath   = """contains(@href,"some_file")"""
found_field    = tree.xpath(simple_xpath)
print("Result2 = {}".format(found_field))

simple_xpath   = """@href[contains(@href,"some_file")]"""
found_field    = tree.xpath(simple_xpath)
print("Result3 = {}".format(found_field))

Actual output:

Result1 = ['some_map/some_file.png']
Result2 = True
Result3 = []

Expected output:

Result1 = ['some_map/some_file.png']
Result2 = True
Result3 = ['some_map/some_file.png']

Answer 1

Your predicate in the third example ( @href[contains(@href,"some_file")] ), translated into English means "find a node in simple_record which has an attribute href which itself has an attribute href which has an attribute value containing the string some_file ". Such node doesn't exist, so an empty result list is returned.

What you intended to ask, in English, is "find a node in simple_record which has an attribute href which has a value containing the string some_file " (Thanks @DanielHaley!) . Translated into xpath, you would write it as

simple_xpath   = '@href[contains(.,"some_file")]'

The . now refers back to the context node that is being filtered by the predicate (ie the @href attribute itself). That expression would cause Result 3 to be the same as Result 1.