• January 23, 2023

Scrapy Middleware Example

Spider Middleware — Scrapy 2.5.1 documentation

Spider Middleware — Scrapy 2.5.1 documentation

The spider middleware is a framework of hooks into Scrapy’s spider processing
mechanism where you can plug custom functionality to process the responses that
are sent to Spiders for processing and to process the requests
and items that are generated from spiders.
Activating a spider middleware¶
To activate a spider middleware component, add it to the
SPIDER_MIDDLEWARES setting, which is a dict whose keys are the
middleware class path and their values are the middleware orders.
Here’s an example:
SPIDER_MIDDLEWARES = {
‘stomSpiderMiddleware’: 543, }
The SPIDER_MIDDLEWARES setting is merged with the
SPIDER_MIDDLEWARES_BASE setting defined in Scrapy (and not meant to
be overridden) and then sorted by order to get the final sorted list of enabled
middlewares: the first middleware is the one closer to the engine and the last
is the one closer to the spider. In other words,
the process_spider_input()
method of each middleware will be invoked in increasing
middleware order (100, 200, 300, …), and the
process_spider_output() method
of each middleware will be invoked in decreasing order.
To decide which order to assign to your middleware see the
SPIDER_MIDDLEWARES_BASE setting and pick a value according to where
you want to insert the middleware. The order does matter because each
middleware performs a different action and your middleware could depend on some
previous (or subsequent) middleware being applied.
If you want to disable a builtin middleware (the ones defined in
SPIDER_MIDDLEWARES_BASE, and enabled by default) you must define it
in your project SPIDER_MIDDLEWARES setting and assign None as its
value. For example, if you want to disable the off-site middleware:
‘stomSpiderMiddleware’: 543,
‘siteMiddleware’: None, }
Finally, keep in mind that some middlewares may need to be enabled through a
particular setting. See each middleware documentation for more info.
Writing your own spider middleware¶
Each spider middleware is a Python class that defines one or more of the
methods defined below.
The main entry point is the from_crawler class method, which receives a
Crawler instance. The Crawler
object gives you access, for example, to the settings.
class scrapy. spidermiddlewares. SpiderMiddleware¶
process_spider_input(response, spider)¶
This method is called for each response that goes through the spider
middleware and into the spider, for processing.
process_spider_input() should return None or raise an
exception.
If it returns None, Scrapy will continue processing this response,
executing all other middlewares until, finally, the response is handed
to the spider for processing.
If it raises an exception, Scrapy won’t bother calling any other spider
middleware process_spider_input() and will call the request
errback if there is one, otherwise it will start the process_spider_exception()
chain. The output of the errback is chained back in the other
direction for process_spider_output() to process it, or
process_spider_exception() if it raised an exception.
Parameters
response (Response object) – the response being processed
spider (Spider object) – the spider for which this response is intended
process_spider_output(response, result, spider)¶
This method is called with the results returned from the Spider, after
it has processed the response.
process_spider_output() must return an iterable of
Request objects and item object.
response (Response object) – the response which generated this output from the
spider
result (an iterable of Request objects and
item object) – the result returned by the spider
spider (Spider object) – the spider whose result is being processed
process_spider_exception(response, exception, spider)¶
This method is called when a spider or process_spider_output()
method (from a previous spider middleware) raises an exception.
process_spider_exception() should return either None or an
iterable of Request objects and item object.
If it returns None, Scrapy will continue processing this exception,
executing any other process_spider_exception() in the following
middleware components, until no middleware components are left and the
exception reaches the engine (where it’s logged and discarded).
If it returns an iterable the process_spider_output() pipeline
kicks in, starting from the next spider middleware, and no other
process_spider_exception() will be called.
response (Response object) – the response being processed when the exception was
raised
exception (Exception object) – the exception raised
spider (Spider object) – the spider which raised the exception
process_start_requests(start_requests, spider)¶
This method is called with the start requests of the spider, and works
similarly to the process_spider_output() method, except that it
doesn’t have a response associated and must return only requests (not
items).
It receives an iterable (in the start_requests parameter) and must
return another iterable of Request objects.
Note
When implementing this method in your spider middleware, you
should always return an iterable (that follows the input one) and
not consume all start_requests iterator because it can be very
large (or even unbounded) and cause a memory overflow. The Scrapy
engine is designed to pull start requests while it has capacity to
process them, so the start requests iterator can be effectively
endless where there is some other condition for stopping the spider
(like a time limit or item/page count).
start_requests (an iterable of Request) – the start requests
spider (Spider object) – the spider to whom the start requests belong
from_crawler(cls, crawler)¶
If present, this classmethod is called to create a middleware instance
from a Crawler. It must return a new instance
of the middleware. Crawler object provides access to all Scrapy core
components like settings and signals; it is a way for middleware to
access them and hook its functionality into Scrapy.
crawler (Crawler object) – crawler that uses this middleware
Built-in spider middleware reference¶
This page describes all spider middleware components that come with Scrapy. For
information on how to use them and how to write your own spider middleware, see
the spider middleware usage guide.
For a list of the components enabled by default (and their orders) see the
SPIDER_MIDDLEWARES_BASE setting.
DepthMiddleware¶
class [source]¶
DepthMiddleware is used for tracking the depth of each Request inside the
site being scraped. It works by setting [‘depth’] = 0 whenever
there is no value previously set (usually just the first Request) and
incrementing it by 1 otherwise.
It can be used to limit the maximum depth to scrape, control Request
priority based on their depth, and things like that.
The DepthMiddleware can be configured through the following
settings (see the settings documentation for more info):
DEPTH_LIMIT – The maximum depth that will be allowed to
crawl for any site. If zero, no limit will be imposed.
DEPTH_STATS_VERBOSE – Whether to collect the number of
requests for each depth.
DEPTH_PRIORITY – Whether to prioritize the requests based on
their depth.
HttpErrorMiddleware¶
class tpErrorMiddleware[source]¶
Filter out unsuccessful (erroneous) HTTP responses so that spiders don’t
have to deal with them, which (most of the time) imposes an overhead,
consumes more resources, and makes the spider logic more complex.
According to the HTTP standard, successful responses are those whose
status codes are in the 200-300 range.
If you still want to process response codes outside that range, you can
specify which response codes the spider is able to handle using the
handle_tatus_list spider attribute or
HTTPERROR_ALLOWED_CODES setting.
For example, if you want your spider to handle 404 responses you can do
this:
class MySpider(CrawlSpider):
handle_tatus_list = [404]
The handle_tatus_list key of can also be used to specify which response codes to
allow on a per-request basis. You can also set the meta key handle_tatus_all
to True if you want to allow any response code for a request, and False to
disable the effects of the handle_tatus_all key.
Keep in mind, however, that it’s usually a bad idea to handle non-200
responses, unless you really know what you’re doing.
For more information see: HTTP Status Code Definitions.
HttpErrorMiddleware settings¶
HTTPERROR_ALLOWED_CODES¶
Default: []
Pass all responses with non-200 status codes contained in this list.
HTTPERROR_ALLOW_ALL¶
Default: False
Pass all responses, regardless of its status code.
OffsiteMiddleware¶
class siteMiddleware[source]¶
Filters out Requests for URLs outside the domains covered by the spider.
This middleware filters out every request whose host names aren’t in the
spider’s allowed_domains attribute.
All subdomains of any domain in the list are also allowed.
E. g. the rule will also allow not nor
When your spider returns a request for a domain not belonging to those
covered by the spider, this middleware will log a debug message similar to
this one:
DEBUG: Filtered offsite request to ”:
To avoid filling the log with too much noise, it will only print one of
these messages for each new domain filtered. So, for example, if another
request for is filtered, no log message will be
printed. But if a request for is filtered, a message
will be printed (but only for the first request filtered).
If the spider doesn’t define an
allowed_domains attribute, or the
attribute is empty, the offsite middleware will allow all requests.
If the request has the dont_filter attribute
set, the offsite middleware will allow the request even if its domain is not
listed in allowed domains.
RefererMiddleware¶
class fererMiddleware[source]¶
Populates Request Referer header, based on the URL of the Response which
generated it.
RefererMiddleware settings¶
REFERER_ENABLED¶
Default: True
Whether to enable referer middleware.
REFERRER_POLICY¶
Default: ‘faultReferrerPolicy’
Referrer Policy to apply when populating Request “Referer” header.
You can also set the Referrer Policy per request,
using the special “referrer_policy” key,
with the same acceptable values as for the REFERRER_POLICY setting.
Acceptable values for REFERRER_POLICY¶
either a path to a ferrerPolicy
subclass — a custom policy or one of the built-in ones (see classes below),
or one of the standard W3C-defined string values,
or the special “scrapy-default”.
String value
Class name (as a string)
“scrapy-default” (default)
faultReferrerPolicy
“no-referrer”
ReferrerPolicy
“no-referrer-when-downgrade”
ReferrerWhenDowngradePolicy
“same-origin”
meOriginPolicy
“origin”
ferer. OriginPolicy
“strict-origin”
rictOriginPolicy
“origin-when-cross-origin”
ferer. OriginWhenCrossOriginPolicy
“strict-origin-when-cross-origin”
rictOriginWhenCrossOriginPolicy
“unsafe-url”
ferer. UnsafeUrlPolicy
class faultReferrerPolicy[source]¶
A variant of “no-referrer-when-downgrade”,
with the addition that “Referer” is not sent if the parent request was
using file or s3 scheme.
Warning
Scrapy’s default referrer policy — just like “no-referrer-when-downgrade”,
the W3C-recommended value for browsers — will send a non-empty
“Referer” header from any (s) to any URL,
even if the domain is different.
“same-origin” may be a better choice if you want to remove referrer
information for cross-domain requests.
class ReferrerPolicy[source]¶
The simplest policy is “no-referrer”, which specifies that no referrer information
is to be sent along with requests made from a particular request client to any origin.
The header will be omitted entirely.
class ReferrerWhenDowngradePolicy[source]¶
The “no-referrer-when-downgrade” policy sends a full URL along with requests
from a TLS-protected environment settings object to a potentially trustworthy URL,
and requests from clients which are not TLS-protected to any origin.
Requests from TLS-protected clients to non-potentially trustworthy URLs,
on the other hand, will contain no referrer information.
A Referer HTTP header will not be sent.
This is a user agent’s default behavior, if no policy is otherwise specified.
“no-referrer-when-downgrade” policy is the W3C-recommended default,
and is used by major web browsers.
However, it is NOT Scrapy’s default referrer policy (see DefaultReferrerPolicy).
class meOriginPolicy[source]¶
The “same-origin” policy specifies that a full URL, stripped for use as a referrer,
is sent as referrer information when making same-origin requests from a particular request client.
Cross-origin requests, on the other hand, will contain no referrer information.
class ferer. OriginPolicy[source]¶
The “origin” policy specifies that only the ASCII serialization
of the origin of the request client is sent as referrer information
when making both same-origin requests and cross-origin requests
from a particular request client.
class rictOriginPolicy[source]¶
The “strict-origin” policy sends the ASCII serialization
of the origin of the request client when making requests:
– from a TLS-protected environment settings object to a potentially trustworthy URL, and
– from non-TLS-protected environment settings objects to any origin.
Requests from TLS-protected request clients to non- potentially trustworthy URLs,
class ferer. OriginWhenCrossOriginPolicy[source]¶
The “origin-when-cross-origin” policy specifies that a full URL,
stripped for use as a referrer, is sent as referrer information
when making same-origin requests from a particular request client,
and only the ASCII serialization of the origin of the request client
is sent as referrer information when making cross-origin requests
class rictOriginWhenCrossOriginPolicy[source]¶
The “strict-origin-when-cross-origin” policy specifies that a full URL,
when making cross-origin requests:
from a TLS-protected environment settings object to a potentially trustworthy URL, and
from non-TLS-protected environment settings objects to any origin.
Requests from TLS-protected clients to non- potentially trustworthy URLs,
class ferer. UnsafeUrlPolicy[source]¶
The “unsafe-url” policy specifies that a full URL, stripped for use as a referrer,
is sent along with both cross-origin requests
and same-origin requests made from a particular request client.
Note: The policy’s name doesn’t lie; it is unsafe.
This policy will leak origins and paths from TLS-protected resources
to insecure origins.
Carefully consider the impact of setting such a policy for potentially sensitive documents.
“unsafe-url” policy is NOT recommended.
UrlLengthMiddleware¶
class scrapy. urllength. UrlLengthMiddleware[source]¶
Filters out requests with URLs longer than URLLENGTH_LIMIT
The UrlLengthMiddleware can be configured through the following
URLLENGTH_LIMIT – The maximum URL length to allow for crawled URLs.
Spider Middleware — Scrapy 2.5.1 documentation

Spider Middleware — Scrapy 2.5.1 documentation

The spider middleware is a framework of hooks into Scrapy’s spider processing
mechanism where you can plug custom functionality to process the responses that
are sent to Spiders for processing and to process the requests
and items that are generated from spiders.
Activating a spider middleware¶
To activate a spider middleware component, add it to the
SPIDER_MIDDLEWARES setting, which is a dict whose keys are the
middleware class path and their values are the middleware orders.
Here’s an example:
SPIDER_MIDDLEWARES = {
‘stomSpiderMiddleware’: 543, }
The SPIDER_MIDDLEWARES setting is merged with the
SPIDER_MIDDLEWARES_BASE setting defined in Scrapy (and not meant to
be overridden) and then sorted by order to get the final sorted list of enabled
middlewares: the first middleware is the one closer to the engine and the last
is the one closer to the spider. In other words,
the process_spider_input()
method of each middleware will be invoked in increasing
middleware order (100, 200, 300, …), and the
process_spider_output() method
of each middleware will be invoked in decreasing order.
To decide which order to assign to your middleware see the
SPIDER_MIDDLEWARES_BASE setting and pick a value according to where
you want to insert the middleware. The order does matter because each
middleware performs a different action and your middleware could depend on some
previous (or subsequent) middleware being applied.
If you want to disable a builtin middleware (the ones defined in
SPIDER_MIDDLEWARES_BASE, and enabled by default) you must define it
in your project SPIDER_MIDDLEWARES setting and assign None as its
value. For example, if you want to disable the off-site middleware:
‘stomSpiderMiddleware’: 543,
‘siteMiddleware’: None, }
Finally, keep in mind that some middlewares may need to be enabled through a
particular setting. See each middleware documentation for more info.
Writing your own spider middleware¶
Each spider middleware is a Python class that defines one or more of the
methods defined below.
The main entry point is the from_crawler class method, which receives a
Crawler instance. The Crawler
object gives you access, for example, to the settings.
class scrapy. spidermiddlewares. SpiderMiddleware¶
process_spider_input(response, spider)¶
This method is called for each response that goes through the spider
middleware and into the spider, for processing.
process_spider_input() should return None or raise an
exception.
If it returns None, Scrapy will continue processing this response,
executing all other middlewares until, finally, the response is handed
to the spider for processing.
If it raises an exception, Scrapy won’t bother calling any other spider
middleware process_spider_input() and will call the request
errback if there is one, otherwise it will start the process_spider_exception()
chain. The output of the errback is chained back in the other
direction for process_spider_output() to process it, or
process_spider_exception() if it raised an exception.
Parameters
response (Response object) – the response being processed
spider (Spider object) – the spider for which this response is intended
process_spider_output(response, result, spider)¶
This method is called with the results returned from the Spider, after
it has processed the response.
process_spider_output() must return an iterable of
Request objects and item object.
response (Response object) – the response which generated this output from the
spider
result (an iterable of Request objects and
item object) – the result returned by the spider
spider (Spider object) – the spider whose result is being processed
process_spider_exception(response, exception, spider)¶
This method is called when a spider or process_spider_output()
method (from a previous spider middleware) raises an exception.
process_spider_exception() should return either None or an
iterable of Request objects and item object.
If it returns None, Scrapy will continue processing this exception,
executing any other process_spider_exception() in the following
middleware components, until no middleware components are left and the
exception reaches the engine (where it’s logged and discarded).
If it returns an iterable the process_spider_output() pipeline
kicks in, starting from the next spider middleware, and no other
process_spider_exception() will be called.
response (Response object) – the response being processed when the exception was
raised
exception (Exception object) – the exception raised
spider (Spider object) – the spider which raised the exception
process_start_requests(start_requests, spider)¶
This method is called with the start requests of the spider, and works
similarly to the process_spider_output() method, except that it
doesn’t have a response associated and must return only requests (not
items).
It receives an iterable (in the start_requests parameter) and must
return another iterable of Request objects.
Note
When implementing this method in your spider middleware, you
should always return an iterable (that follows the input one) and
not consume all start_requests iterator because it can be very
large (or even unbounded) and cause a memory overflow. The Scrapy
engine is designed to pull start requests while it has capacity to
process them, so the start requests iterator can be effectively
endless where there is some other condition for stopping the spider
(like a time limit or item/page count).
start_requests (an iterable of Request) – the start requests
spider (Spider object) – the spider to whom the start requests belong
from_crawler(cls, crawler)¶
If present, this classmethod is called to create a middleware instance
from a Crawler. It must return a new instance
of the middleware. Crawler object provides access to all Scrapy core
components like settings and signals; it is a way for middleware to
access them and hook its functionality into Scrapy.
crawler (Crawler object) – crawler that uses this middleware
Built-in spider middleware reference¶
This page describes all spider middleware components that come with Scrapy. For
information on how to use them and how to write your own spider middleware, see
the spider middleware usage guide.
For a list of the components enabled by default (and their orders) see the
SPIDER_MIDDLEWARES_BASE setting.
DepthMiddleware¶
class [source]¶
DepthMiddleware is used for tracking the depth of each Request inside the
site being scraped. It works by setting [‘depth’] = 0 whenever
there is no value previously set (usually just the first Request) and
incrementing it by 1 otherwise.
It can be used to limit the maximum depth to scrape, control Request
priority based on their depth, and things like that.
The DepthMiddleware can be configured through the following
settings (see the settings documentation for more info):
DEPTH_LIMIT – The maximum depth that will be allowed to
crawl for any site. If zero, no limit will be imposed.
DEPTH_STATS_VERBOSE – Whether to collect the number of
requests for each depth.
DEPTH_PRIORITY – Whether to prioritize the requests based on
their depth.
HttpErrorMiddleware¶
class tpErrorMiddleware[source]¶
Filter out unsuccessful (erroneous) HTTP responses so that spiders don’t
have to deal with them, which (most of the time) imposes an overhead,
consumes more resources, and makes the spider logic more complex.
According to the HTTP standard, successful responses are those whose
status codes are in the 200-300 range.
If you still want to process response codes outside that range, you can
specify which response codes the spider is able to handle using the
handle_tatus_list spider attribute or
HTTPERROR_ALLOWED_CODES setting.
For example, if you want your spider to handle 404 responses you can do
this:
class MySpider(CrawlSpider):
handle_tatus_list = [404]
The handle_tatus_list key of can also be used to specify which response codes to
allow on a per-request basis. You can also set the meta key handle_tatus_all
to True if you want to allow any response code for a request, and False to
disable the effects of the handle_tatus_all key.
Keep in mind, however, that it’s usually a bad idea to handle non-200
responses, unless you really know what you’re doing.
For more information see: HTTP Status Code Definitions.
HttpErrorMiddleware settings¶
HTTPERROR_ALLOWED_CODES¶
Default: []
Pass all responses with non-200 status codes contained in this list.
HTTPERROR_ALLOW_ALL¶
Default: False
Pass all responses, regardless of its status code.
OffsiteMiddleware¶
class siteMiddleware[source]¶
Filters out Requests for URLs outside the domains covered by the spider.
This middleware filters out every request whose host names aren’t in the
spider’s allowed_domains attribute.
All subdomains of any domain in the list are also allowed.
E. g. the rule will also allow not nor
When your spider returns a request for a domain not belonging to those
covered by the spider, this middleware will log a debug message similar to
this one:
DEBUG: Filtered offsite request to ”:
To avoid filling the log with too much noise, it will only print one of
these messages for each new domain filtered. So, for example, if another
request for is filtered, no log message will be
printed. But if a request for is filtered, a message
will be printed (but only for the first request filtered).
If the spider doesn’t define an
allowed_domains attribute, or the
attribute is empty, the offsite middleware will allow all requests.
If the request has the dont_filter attribute
set, the offsite middleware will allow the request even if its domain is not
listed in allowed domains.
RefererMiddleware¶
class fererMiddleware[source]¶
Populates Request Referer header, based on the URL of the Response which
generated it.
RefererMiddleware settings¶
REFERER_ENABLED¶
Default: True
Whether to enable referer middleware.
REFERRER_POLICY¶
Default: ‘faultReferrerPolicy’
Referrer Policy to apply when populating Request “Referer” header.
You can also set the Referrer Policy per request,
using the special “referrer_policy” key,
with the same acceptable values as for the REFERRER_POLICY setting.
Acceptable values for REFERRER_POLICY¶
either a path to a ferrerPolicy
subclass — a custom policy or one of the built-in ones (see classes below),
or one of the standard W3C-defined string values,
or the special “scrapy-default”.
String value
Class name (as a string)
“scrapy-default” (default)
faultReferrerPolicy
“no-referrer”
ReferrerPolicy
“no-referrer-when-downgrade”
ReferrerWhenDowngradePolicy
“same-origin”
meOriginPolicy
“origin”
ferer. OriginPolicy
“strict-origin”
rictOriginPolicy
“origin-when-cross-origin”
ferer. OriginWhenCrossOriginPolicy
“strict-origin-when-cross-origin”
rictOriginWhenCrossOriginPolicy
“unsafe-url”
ferer. UnsafeUrlPolicy
class faultReferrerPolicy[source]¶
A variant of “no-referrer-when-downgrade”,
with the addition that “Referer” is not sent if the parent request was
using file or s3 scheme.
Warning
Scrapy’s default referrer policy — just like “no-referrer-when-downgrade”,
the W3C-recommended value for browsers — will send a non-empty
“Referer” header from any (s) to any URL,
even if the domain is different.
“same-origin” may be a better choice if you want to remove referrer
information for cross-domain requests.
class ReferrerPolicy[source]¶
The simplest policy is “no-referrer”, which specifies that no referrer information
is to be sent along with requests made from a particular request client to any origin.
The header will be omitted entirely.
class ReferrerWhenDowngradePolicy[source]¶
The “no-referrer-when-downgrade” policy sends a full URL along with requests
from a TLS-protected environment settings object to a potentially trustworthy URL,
and requests from clients which are not TLS-protected to any origin.
Requests from TLS-protected clients to non-potentially trustworthy URLs,
on the other hand, will contain no referrer information.
A Referer HTTP header will not be sent.
This is a user agent’s default behavior, if no policy is otherwise specified.
“no-referrer-when-downgrade” policy is the W3C-recommended default,
and is used by major web browsers.
However, it is NOT Scrapy’s default referrer policy (see DefaultReferrerPolicy).
class meOriginPolicy[source]¶
The “same-origin” policy specifies that a full URL, stripped for use as a referrer,
is sent as referrer information when making same-origin requests from a particular request client.
Cross-origin requests, on the other hand, will contain no referrer information.
class ferer. OriginPolicy[source]¶
The “origin” policy specifies that only the ASCII serialization
of the origin of the request client is sent as referrer information
when making both same-origin requests and cross-origin requests
from a particular request client.
class rictOriginPolicy[source]¶
The “strict-origin” policy sends the ASCII serialization
of the origin of the request client when making requests:
– from a TLS-protected environment settings object to a potentially trustworthy URL, and
– from non-TLS-protected environment settings objects to any origin.
Requests from TLS-protected request clients to non- potentially trustworthy URLs,
class ferer. OriginWhenCrossOriginPolicy[source]¶
The “origin-when-cross-origin” policy specifies that a full URL,
stripped for use as a referrer, is sent as referrer information
when making same-origin requests from a particular request client,
and only the ASCII serialization of the origin of the request client
is sent as referrer information when making cross-origin requests
class rictOriginWhenCrossOriginPolicy[source]¶
The “strict-origin-when-cross-origin” policy specifies that a full URL,
when making cross-origin requests:
from a TLS-protected environment settings object to a potentially trustworthy URL, and
from non-TLS-protected environment settings objects to any origin.
Requests from TLS-protected clients to non- potentially trustworthy URLs,
class ferer. UnsafeUrlPolicy[source]¶
The “unsafe-url” policy specifies that a full URL, stripped for use as a referrer,
is sent along with both cross-origin requests
and same-origin requests made from a particular request client.
Note: The policy’s name doesn’t lie; it is unsafe.
This policy will leak origins and paths from TLS-protected resources
to insecure origins.
Carefully consider the impact of setting such a policy for potentially sensitive documents.
“unsafe-url” policy is NOT recommended.
UrlLengthMiddleware¶
class scrapy. urllength. UrlLengthMiddleware[source]¶
Filters out requests with URLs longer than URLLENGTH_LIMIT
The UrlLengthMiddleware can be configured through the following
URLLENGTH_LIMIT – The maximum URL length to allow for crawled URLs.
Architecture overview — Scrapy 2.5.1 documentation

Architecture overview — Scrapy 2.5.1 documentation

This document describes the architecture of Scrapy and how its components
interact.
Overview¶
The following diagram shows an overview of the Scrapy architecture with its
components and an outline of the data flow that takes place inside the system
(shown by the red arrows). A brief description of the components is included
below with links for more detailed information about them. The data flow is
also described below.
Data flow¶
The data flow in Scrapy is controlled by the execution engine, and goes like
this:
The Engine gets the initial Requests to crawl from the
Spider.
The Engine schedules the Requests in the
Scheduler and asks for the
next Requests to crawl.
The Scheduler returns the next Requests
to the Engine.
The Engine sends the Requests to the
Downloader, passing through the
Downloader Middlewares (see
process_request()).
Once the page finishes downloading the
Downloader generates a Response (with
that page) and sends it to the Engine, passing through the
process_response()).
The Engine receives the Response from the
Downloader and sends it to the
Spider for processing, passing
through the Spider Middleware (see
process_spider_input()).
The Spider processes the Response and returns
scraped items and new Requests (to follow) to the
Engine, passing through the
Spider Middleware (see
process_spider_output()).
The Engine sends processed items to
Item Pipelines, then send processed Requests to
the Scheduler and asks for possible next Requests
to crawl.
The process repeats (from step 1) until there are no more requests from the
Scheduler.
Components¶
Scrapy Engine¶
The engine is responsible for controlling the data flow between all components
of the system, and triggering events when certain actions occur. See the
Data Flow section above for more details.
Scheduler¶
The Scheduler receives requests from the engine and enqueues them for feeding
them later (also to the engine) when the engine requests them.
Downloader¶
The Downloader is responsible for fetching web pages and feeding them to the
engine which, in turn, feeds them to the spiders.
Spiders¶
Spiders are custom classes written by Scrapy users to parse responses and
extract items from them or additional requests to
follow. For more information see Spiders.
Item Pipeline¶
The Item Pipeline is responsible for processing the items once they have been
extracted (or scraped) by the spiders. Typical tasks include cleansing,
validation and persistence (like storing the item in a database). For more
information see Item Pipeline.
Downloader middlewares¶
Downloader middlewares are specific hooks that sit between the Engine and the
Downloader and process requests when they pass from the Engine to the
Downloader, and responses that pass from Downloader to the Engine.
Use a Downloader middleware if you need to do one of the following:
process a request just before it is sent to the Downloader
(i. e. right before Scrapy sends the request to the website);
change received response before passing it to a spider;
send a new Request instead of passing received response to a spider;
pass response to a spider without fetching a web page;
silently drop some requests.
For more information see Downloader Middleware.
Spider middlewares¶
Spider middlewares are specific hooks that sit between the Engine and the
Spiders and are able to process spider input (responses) and output (items and
requests).
Use a Spider middleware if you need to
post-process output of spider callbacks – change/add/remove requests or items;
post-process start_requests;
handle spider exceptions;
call errback instead of callback for some of the requests based on response
content.
For more information see Spider Middleware.
Event-driven networking¶
Scrapy is written with Twisted, a popular event-driven networking framework
for Python. Thus, it’s implemented using a non-blocking (aka asynchronous) code
for concurrency.
For more information about asynchronous programming and Twisted see these
links:
Introduction to Deferreds
Twisted – hello, asynchronous programming
Twisted Introduction – Krondo

Frequently Asked Questions about scrapy middleware example

What is Scrapy middleware?

The spider middleware is a framework of hooks into Scrapy’s spider processing mechanism where you can plug custom functionality to process the responses that are sent to Spiders for processing and to process the requests and items that are generated from spiders.

How does a Scrapy pipeline work?

The data flow in Scrapy is controlled by the execution engine, and goes like this: The Engine gets the initial Requests to crawl from the Spider. … The Engine sends processed items to Item Pipelines, then send processed Requests to the Scheduler and asks for possible next Requests to crawl.

How does Scrapy spider work?

Scrapy provides Item pipelines that allow you to write functions in your spider that can process your data such as validating data, removing data and saving data to a database. It provides spider Contracts to test your spiders and allows you to create generic and deep crawlers as well.Jan 11, 2019

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