Over a million developers have joined DZone.
{{announcement.body}}
{{announcement.title}}

Escaping Solr Query Characters In Python

DZone's Guide to

Escaping Solr Query Characters In Python

· Big Data Zone
Free Resource

Effortlessly power IoT, predictive analytics, and machine learning applications with an elastic, resilient data infrastructure. Learn how with Mesosphere DC/OS.

I’ve been working in some Python Solr client code. One area where bugs have cropped up is in query terms that need to be escaped before passing to Solr. For example, how do you send Solr an argument term with a “:” in it? Or a “(“?

It turns out that generally you just put a \ in front of the character you want to escape. So to search for “:” in the “funnychars” field, you would send q=funnychars:\:.

Php programmer Mats Lindh has solved this pretty well, using str_replace. str_replace is a convenient, general-purpose string replacement function that lets you do batch string replacement. For example you can do:


$matches = array("Mary","lamb","fleece");
$replacements = array("Harry","dog","fur");
str_replace($matches, $replacements,"Mary had a little lamb, its fleece was white as snow");

Python doesn’t quite have str_replace. There is translate which does single character to single character batch replacement. That can’t be used for escaping because the destination values are strings(ie “\:”), not single characters. There’s a general purpose “str_replace” drop-in replacement at this Stackoverflow question:


edits =[("Mary","Harry"),("lamb","dog"),("fleece","fur")]# etc.for search, replace in edits:
  s = s.replace(search, replace)

You’ll notice that this algorithm requires multiple passes through the string for search/replacement. This is because that earlier search/replaces may impact later search/replaces. For example, what if edits was this:


edits =[("Mary","Harry"),("Harry","Larry"),("Larry","Barry")]

First our str_replace will replace Mary with Harry in pass 1, then Harry with Larry in pass 2, etc.

It turns out that escaping characters is a narrower string replacement case that can be done more efficiently without too much complication. The only character that one needs to worry about impacting other rules is escaping the \, as the other rules insert \ characters, we wouldn’t want them double escaped.

Aside from this caveat, all the escaping rules can be processed from a single pass through the string which my solution below does, performing a heck of a lot faster:


# These rules all independent, order of# escaping doesn't matter
escapeRules ={'+':r'\+','-':r'\-','&':r'\&','|':r'\|','!':r'\!','(':r'\(',')':r'\)','{':r'\{','}':r'\}','[':r'\[',']':r'\]','^':r'\^','~':r'\~','*':r'\*','?':r'\?',':':r'\:','"':r'\"',';':r'\;',' ':r'\ '}defescapedSeq(term):""" Yield the next string based on the        next character (either this char        or escaped version """forcharin term:ifcharin escapeRules.keys():yield escapeRules[char]else:yieldchardefescapeSolrArg(term):""" Apply escaping to the passed in query terms        escaping special characters like : , etc"""
    term = term.replace('\\',r'\\')# escape \ firstreturn"".join([nextStr for nextStr in escapedSeq(term)])

Aside from being a good general solution to this problem, in some basic benchmarks, this turns out to be about 5 orders of magnitude faster than doing it the naive way! Pretty cool, but you’ll probably rarely notice the difference. Nevertheless it could matter in specialized cases if you are automatically constructing and batching large/gnarly queries that require a lot of work to escape.

Anyway, enjoy! I’d love to hear what you think!

Learn to design and build better data-rich applications with this free eBook from O’Reilly. Brought to you by Mesosphere DC/OS.

Topics:

Published at DZone with permission of Doug Turnbull, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

THE DZONE NEWSLETTER

Dev Resources & Solutions Straight to Your Inbox

Thanks for subscribing!

Awesome! Check your inbox to verify your email so you can start receiving the latest in tech news and resources.

X

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}