# Frequently Asked Questions

Contents

- General

  - Which version of Python do I need?

  - What dependencies does mechanize need?

  - What license does mechanize use?

- Usage

  - I’m not getting the HTML page I expected to see?

  - Is JavaScript supported?

  - My HTTP response data is truncated?

  - Is there any example code?

- Cookies

  - Which HTTP cookie protocols does mechanize support?

  - What about RFC 2109?

  - Why don’t I have any cookies?

  - My response claims to be empty, but I know it’s not?

  - What’s the difference between the .load() and .revert() methods of CookieJar?

  - Is it threadsafe?

  - How do I do X?

- Forms

  - How do I figure out what control names and values to use?

  - What do those ‘*’ characters mean in the string representations of list controls?

  - What do those parentheses (round brackets) mean in the string representations of list controls?

  - Why doesn’t <some control> turn up in the data returned by .click*() when that control has non-None value?

  - Why does mechanize not follow the HTML 4.0 / RFC 1866 standards for RADIO and multiple-selection SELECT controls?

  - Why does .click() ing on a button not work for me?

  - How do I change INPUT TYPE=HIDDEN field values (for example, to emulate the effect of JavaScript code)?

  - I’m having trouble debugging my code.

  - I have a control containing a list of integers. How do I select the one whose value is nearest to the one I want?

- Miscellaneous

  - I want to see what my web browser is doing?

  - JavaScript is messing up my web-scraping. What do I do?

## General

### Which version of Python do I need?

mechanize works on all python versions, python 2 (>= 2.7) and 3 (>= 3.5).

### What dependencies does mechanize need?

    html5lib

### What license does mechanize use?

mechanize is licensed under the BSD-3-clause license.

## Usage

### I’m not getting the HTML page I expected to see?

See Debugging.

### Is JavaScript supported?

No, sorry. See JavaScript is messing up my web-scraping. What do I do?

### My HTTP response data is truncated?

mechanize.Browser’s response objects support the .seek() method, and can still be used after .close() has been called. Response data is not fetched until it is needed, so navigation away from a URL before fetching all of the response will truncate it. Call response.get_data() before navigation if you don’t want that to happen.

### Is there any example code?

Look in the examples/ directory. Note that the examples on the forms page are executable as-is. Contributions of example code would be very welcome!

## Cookies

### Which HTTP cookie protocols does mechanize support?

Netscape and RFC 2965. RFC 2965 handling is switched off by default.

### What about RFC 2109?

RFC 2109 cookies are currently parsed as Netscape cookies, and treated by default as RFC 2965 cookies thereafter if RFC 2965 handling is enabled, or as Netscape cookies otherwise.

### Why don’t I have any cookies?

See Cookies.

### My response claims to be empty, but I know it’s not?

Did you call response.read() (e.g., in a debug statement), then forget that all the data has already been read? In that case, you may want to use mechanize.response_seek_wrapper. mechanize.Browser always returns seekable responses, so it’s not necessary to use this explicitly in that case.

### What’s the difference between the .load() and .revert() methods of CookieJar?

.load() appends cookies from a file. .revert() discards all existing cookies held by the CookieJar first (but it won’t lose any existing cookies if the loading fails).

### Is it threadsafe?

See Thread safety.

### How do I do X?

Refer to the API documentation in Browser API.

## Forms

### How do I figure out what control names and values to use?

print(form) is usually all you need. In your code, things like the HTMLForm.items attribute of `mechanize.HTMLForm` instances can be useful to inspect forms at runtime. Note that it’s possible to use item labels instead of item names, which can be useful — use the by_label arguments to the various methods, and the .get_value_by_label() / .set_value_by_label() methods on ListControl.

### What do those ‘*’ characters mean in the string representations of list controls?

A * next to an item means that item is selected.

### What do those parentheses (round brackets) mean in the string representations of list controls?

Parentheses (foo) around an item mean that item is disabled.

### Why doesn’t <some control> turn up in the data returned by .click*() when that control has non-None value?

Either the control is disabled, or it is not successful for some other reason. ‘Successful’ (see HTML 4 specification) means that the control will cause data to get sent to the server.

### Why does mechanize not follow the HTML 4.0 / RFC 1866 standards for RADIO and multiple-selection SELECT controls?

Because by default, it follows browser behaviour when setting the initially-selected items in list controls that have no items explicitly selected in the HTML.

### Why does .click() ing on a button not work for me?

Clicking on a RESET button doesn’t do anything, by design - this is a library for web automation, not an interactive browser. Even in an interactive browser, clicking on RESET sends nothing to the server, so there is little point in having .click() do anything special here.

Clicking on a BUTTON TYPE=BUTTON doesn’t do anything either, also by design. This time, the reason is that that BUTTON is only in the HTML standard so that one can attach JavaScript callbacks to its events. Their execution may result in information getting sent back to the server. mechanize, however, knows nothing about these callbacks, so it can’t do anything useful with a click on a BUTTON whose type is BUTTON.

Generally, JavaScript may be messing things up in all kinds of ways. See JavaScript is messing up my web-scraping. What do I do?.

### How do I change INPUT TYPE=HIDDEN field values (for example, to emulate the effect of JavaScript code)?

As with any control, set the control’s readonly attribute false.

    form.find_control("foo").readonly = False # allow changing .value of control foo
    form.set_all_readonly(False) # allow changing the .value of all controls

### I’m having trouble debugging my code.

See Debugging.

### I have a control containing a list of integers. How do I select the one whose value is nearest to the one I want?

    import bisect
    def closest_int_value(form, ctrl_name, value):
        values = map(int, [item.name for item in form.find_control(ctrl_name).items])
        return str(values[bisect.bisect(values, value) - 1])

    form["distance"] = [closest_int_value(form, "distance", 23)]

## Miscellaneous

### I want to see what my web browser is doing?

Use the developer tools for your browser (you may have to install them first). These provide excellent views into all HTTP requests/responses in the browser.

### JavaScript is messing up my web-scraping. What do I do?

JavaScript is used in web pages for many purposes – for example: creating content that was not present in the page at load time, submitting or filling in parts of forms in response to user actions, setting cookies, etc. mechanize does not provide any support for JavaScript.

If you come across this in a page you want to automate, you have a few options. Here they are, roughly in order of simplicity:

> - Figure out what the JavaScript is doing and emulate it in your Python code. The simplest case is if the JavaScript is setting some cookies. In that case you can inspect the cookies in your browser and emulate setting them in mechanize with `mechanize.Browser.set_simple_cookie()`.

> - More complex is to use your browser developer tools to see exactly what requests are sent by the browser and emulate them in mechanize by using `mechanize.Request` to create the request manually and open it with `mechanize.Browser.open()`.

> - Third is to use some browser automation framework/library to scrape the site instead of using mechanize. These libraries typically drive a headless version of a full browser that can execute all JavaScript. They are typically much slower than using mechanize and far more resource intensive, but do work as a last resort.