# Advanced topics

## Thread safety

The global `mechanize.urlopen()` and `mechanize.urlretrieve()` functions are thread safe. However, mechanize browser instances are not thread safe. If you want to use a mechanize Browser instance in multiple threads, clone it, using copy.copy(browser_object) method. The clone will share the same, thread safe cookie jar, and have the same settings/handlers as the original, but all other state is not shared, making the clone safe to use in a different thread.

## Using custom CA certificates

mechanize supports the same mechanism for using custom CA certificates as python >= 2.7.9. To change the certificates a mechanize browser instance uses, call the `mechanize.Browser.set_ca_data()` method on it.

## Debugging

Hints for debugging programs that use mechanize.

### Cookies

A common mistake is to use `mechanize.urlopen()`, and the .extract_cookies() and .add_cookie_header() methods on a cookie object themselves. If you use mechanize.urlopen() (or OpenerDirector.open()), the module handles extraction and adding of cookies by itself, so you should not call .extract_cookies() or .add_cookie_header().

Are you sure the server is sending you any cookies in the first place? Maybe the server is keeping track of state in some other way (HIDDEN HTML form entries (possibly in a separate page referenced by a frame), URL-encoded session keys, IP address, HTTP Referer headers)? Perhaps some embedded script in the HTML is setting cookies (see below)? Turn on Logging.

When you .save() to or .load()/.revert() from a file, single-session cookies will expire unless you explicitly request otherwise with the ignore_discard argument. This may be your problem if you find cookies are going away after saving and loading.

    import mechanize
    cj = mechanize.LWPCookieJar()
    opener = mechanize.build_opener(mechanize.HTTPCookieProcessor(cj))
    mechanize.install_opener(opener)
    r = mechanize.urlopen("http://foobar.com/")
    cj.save("/some/file", ignore_discard=True, ignore_expires=True)

JavaScript code can set cookies; mechanize does not support this. See JavaScript is messing up my web-scraping. What do I do?.

### General

Enable Logging.

Sometimes, a server wants particular HTTP headers set to the values it expects. For example, the User-Agent header may need to be set (`mechanize.Browser.set_header()`) to a value like that of a popular browser.

Check that the browser is able to do manually what you’re trying to achieve programmatically. Make sure that what you do manually is exactly the same as what you’re trying to do from Python – you may simply be hitting a server bug that only gets revealed if you view pages in a particular order, for example.

Try comparing the headers and data that your program sends with those that a browser sends. Often this will give you the clue you need. You can use the developer tools in any browser to see exactly what the browser sends and receives.

If nothing is obviously wrong with the requests your program is sending and you’re out of ideas, you can reliably locate the problem by copying the headers that a browser sends, and then changing headers until your program stops working again. Temporarily switch to explicitly sending individual HTTP headers (by calling .add_header(), or by using httplib directly). Start by sending exactly the headers that Firefox or Chrome send. You may need to make sure that a valid session ID is sent – the one you got from your browser may no longer be valid. If that works, you can begin the tedious process of changing your headers and data until they match what your original code was sending. You should end up with a minimal set of changes. If you think that reveals a bug in mechanize, please report it.

### Logging

To enable logging to stdout:

    import sys, logging
    logger = logging.getLogger("mechanize")
    logger.addHandler(logging.StreamHandler(sys.stdout))
    logger.setLevel(logging.DEBUG)

You can reduce the amount of information shown by setting the level to logging.INFO instead of logging.DEBUG, or by only enabling logging for one of the following logger names instead of “mechanize”:

> - “mechanize”: Everything.
> - “mechanize.cookies”: Why particular cookies are accepted or rejected and why they are or are not returned. Requires logging enabled at the DEBUG level.
> - “mechanize.http_responses”: HTTP response body data.
> - “mechanize.http_redirects”: HTTP redirect information.

### HTTP headers

An example showing how to enable printing of HTTP headers to stdout, logging of HTTP response bodies, and logging of information about redirections:

    import sys, logging
    import mechanize

    logger = logging.getLogger("mechanize")
    logger.addHandler(logging.StreamHandler(sys.stdout))
    logger.setLevel(logging.DEBUG)

    browser = mechanize.Browser()
    browser.set_debug_http(True)
    browser.set_debug_responses(True)
    browser.set_debug_redirects(True)
    response = browser.open("http://python.org/")

Alternatively, you can examine request and response objects to see what’s going on. Note that requests may involve “sub-requests” in cases such as redirection, in which case you will not see everything that’s going on just by examining the original request and final response.