# Mako 1.1.0 Documentation

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Release: 1.1.0

# The Mako Runtime Environment

This section describes a little bit about the objects and built-in functions that are available in templates.

## Context

The `Context` is the central object that is created when a template is first executed, and is responsible for handling all communication with the outside world. Within the template environment, it is available via the reserved name `context`. The `Context` includes two major components, one of which is the output buffer, which is a file-like object such as Python’s `StringIO` or similar, and the other a dictionary of variables that can be freely referenced within a template; this dictionary is a combination of the arguments sent to the `render()` function and some built-in variables provided by Mako’s runtime environment.

### The Buffer

The buffer is stored within the `Context`, and writing to it is achieved by calling the `write()` method – in a template this looks like `context.write('some string')`. You usually don’t need to care about this, as all text within a template, as well as all expressions provided by `${}`, automatically send everything to this method. The cases you might want to be aware of its existence are if you are dealing with various filtering/buffering scenarios, which are described in Filtering and Buffering, or if you want to programmatically send content to the output stream, such as within a `<% %>` block.

    <%
        context.write("some programmatic text")
    %>

The actual buffer may or may not be the original buffer sent to the `Context` object, as various filtering/caching scenarios may “push” a new buffer onto the context’s underlying buffer stack. For this reason, just stick with `context.write()` and content will always go to the topmost buffer.

### Context Variables

When your template is compiled into a Python module, the body content is enclosed within a Python function called `render_body`. Other top-level defs defined in the template are defined within their own function bodies which are named after the def’s name with the prefix `render_` (i.e. `render_mydef`). One of the first things that happens within these functions is that all variable names that are referenced within the function which are not defined in some other way (i.e. such as via assignment, module level imports, etc.) are pulled from the `Context` object’s dictionary of variables. This is how you’re able to freely reference variable names in a template which automatically correspond to what was passed into the current `Context`.

- What happens if I reference a variable name that is not in the current context? - The value you get back is a special value called `UNDEFINED`, or if the `strict_undefined=True` flag is used a `NameError` is raised. `UNDEFINED` is just a simple global variable with the class `mako.runtime.Undefined`. The `UNDEFINED` object throws an error when you call `str()` on it, which is what happens if you try to use it in an expression.

- UNDEFINED makes it hard for me to find what name is missing - An alternative is to specify the option `strict_undefined=True` to the `Template` or `TemplateLookup`. This will cause any non-present variables to raise an immediate `NameError` which includes the name of the variable in its message when `render()` is called – `UNDEFINED` is not used.

  New in version 0.3.6.

- Why not just return None? Using `UNDEFINED`, or raising a `NameError` is more explicit and allows differentiation between a value of `None` that was explicitly passed to the `Context` and a value that wasn’t present at all.

- Why raise an exception when you call str() on it ? Why not just return a blank string? - Mako tries to stick to the Python philosophy of “explicit is better than implicit”. In this case, it’s decided that the template author should be made to specifically handle a missing value rather than experiencing what may be a silent failure. Since `UNDEFINED` is a singleton object just like Python’s `True` or `False`, you can use the `is` operator to check for it:

      % if someval is UNDEFINED:
          someval is: no value
      % else:
          someval is: ${someval}
      % endif

Another facet of the `Context` is that its dictionary of variables is immutable. Whatever is set when `render()` is called is what stays. Of course, since its Python, you can hack around this and change values in the context’s internal dictionary, but this will probably will not work as well as you’d think. The reason for this is that Mako in many cases creates copies of the `Context` object, which get sent to various elements of the template and inheriting templates used in an execution. So changing the value in your local `Context` will not necessarily make that value available in other parts of the template’s execution. Examples of where Mako creates copies of the `Context` include within top-level def calls from the main body of the template (the context is used to propagate locally assigned variables into the scope of defs; since in the template’s body they appear as inlined functions, Mako tries to make them act that way), and within an inheritance chain (each template in an inheritance chain has a different notion of `parent` and `next`, which are all stored in unique `Context` instances).

- So what if I want to set values that are global to everyone within a template request? - All you have to do is provide a dictionary to your `Context` when the template first runs, and everyone can just get/set variables from that. Lets say its called `attributes`.

  Running the template looks like:

      output = template.render(attributes={})

  Within a template, just reference the dictionary:

      <%
          attributes['foo'] = 'bar'
      %>
      'foo' attribute is: ${attributes['foo']}

- Why can’t “attributes” be a built-in feature of the Context? - This is an area where Mako is trying to make as few decisions about your application as it possibly can. Perhaps you don’t want your templates to use this technique of assigning and sharing data, or perhaps you have a different notion of the names and kinds of data structures that should be passed around. Once again Mako would rather ask the user to be explicit.

### Context Methods and Accessors

Significant members of `Context` include:

- `context[key]` / `context.get(key, default=None)` - dictionary-like accessors for the context. Normally, any variable you use in your template is automatically pulled from the context if it isn’t defined somewhere already. Use the dictionary accessor and/or `get` method when you want a variable that is already defined somewhere else, such as in the local arguments sent to a `%def` call. If a key is not present, like a dictionary it raises `KeyError`.

- `keys()` - all the names defined within this context.

- `kwargs` - this returns a copy of the context’s dictionary of variables. This is useful when you want to propagate the variables in the current context to a function as keyword arguments, i.e.:

      ${next.body(**context.kwargs)}

- `write(text)` - write some text to the current output stream.

- `lookup` - returns the `TemplateLookup` instance that is used for all file-lookups within the current execution (even though individual `Template` instances can conceivably have different instances of a `TemplateLookup`, only the `TemplateLookup` of the originally-called `Template` gets used in a particular execution).

## The Loop Context

Within `% for` blocks, the reserved name `loop` is available. `loop` tracks the progress of the `for` loop and makes it easy to use the iteration state to control template behavior:

    <ul>
    % for a in ("one", "two", "three"):
        <li>Item ${loop.index}: ${a}</li>
    % endfor
    </ul>

New in version 0.7.

### Iterations

Regardless of the type of iterable you’re looping over, `loop` always tracks the 0-indexed iteration count (available at `loop.index`), its parity (through the `loop.even` and `loop.odd` bools), and `loop.first`, a bool indicating whether the loop is on its first iteration. If your iterable provides a `__len__` method, `loop` also provides access to a count of iterations remaining at `loop.reverse_index` and `loop.last`, a bool indicating whether the loop is on its last iteration; accessing these without `__len__` will raise a `TypeError`.

### Cycling

Cycling is available regardless of whether the iterable you’re using provides a `__len__` method. Prior to Mako 0.7, you might have generated a simple zebra striped list using `enumerate`:

    <ul>
    % for i, item in enumerate(('spam', 'ham', 'eggs')):
      <li class="${'odd' if i % 2 else 'even'}">${item}</li>
    % endfor
    </ul>

With `loop.cycle`, you get the same results with cleaner code and less prep work:

    <ul>
    % for item in ('spam', 'ham', 'eggs'):
      <li class="${loop.cycle('even', 'odd')}">${item}</li>
    % endfor
    </ul>

Both approaches produce output like the following:

    <ul>
      <li class="even">spam</li>
      <li class="odd">ham</li>
      <li class="even">eggs</li>
    </ul>

### Parent Loops

Loop contexts can also be transparently nested, and the Mako runtime will do the right thing and manage the scope for you. You can access the parent loop context through `loop.parent`.

This allows you to reach all the way back up through the loop stack by chaining `parent` attribute accesses, i.e. `loop.parent.parent....` as long as the stack depth isn’t exceeded. For example, you can use the parent loop to make a checkered table:

    <table>
    % for consonant in 'pbj':
      <tr>
      % for vowel in 'iou':
        <td class="${'black' if (loop.parent.even == loop.even) else 'red'}">
          ${consonant + vowel}t
        </td>
      % endfor
      </tr>
    % endfor
    </table>

    <table>
      <tr>
        <td class="black">
          pit
        </td>
        <td class="red">
          pot
        </td>
        <td class="black">
          put
        </td>
      </tr>
      <tr>
        <td class="red">
          bit
        </td>
        <td class="black">
          bot
        </td>
        <td class="red">
          but
        </td>
      </tr>
      <tr>
        <td class="black">
          jit
        </td>
        <td class="red">
          jot
        </td>
        <td class="black">
          jut
        </td>
      </tr>
    </table>

### Migrating Legacy Templates that Use the Word “loop”

Changed in version 0.7: The `loop` name is now reserved in Mako, which means a template that refers to a variable named `loop` won’t function correctly when used in Mako 0.7.

To ease the transition for such systems, the feature can be disabled across the board for all templates, then re-enabled on a per-template basis for those templates which wish to make use of the new system.

First, the `enable_loop=False` flag is passed to either the `TemplateLookup` or `Template` object in use:

    lookup = TemplateLookup(directories=['/docs'], enable_loop=False)

or:

    template = Template("some template", enable_loop=False)

An individual template can make usage of the feature when `enable_loop` is set to `False` by switching it back on within the `<%page>` tag:

    <%page enable_loop="True"/>

    % for i in collection:
        ${i} ${loop.index}
    % endfor

Using the above scheme, it’s safe to pass the name `loop` to the `Template.render()` method as well as to freely make usage of a variable named `loop` within a template, provided the `<%page>` tag doesn’t override it. New templates that want to use the `loop` context can then set up `<%page enable_loop="True"/>` to use the new feature without affecting old templates.

## All the Built-in Names

A one-stop shop for all the names Mako defines. Most of these names are instances of `Namespace`, which are described in the next section, Namespaces. Also, most of these names other than `context`, `UNDEFINED`, and `loop` are also present within the `Context` itself. The names `context`, `loop` and `UNDEFINED` themselves can’t be passed to the context and can’t be substituted – see the section Reserved Names.

- `context` - this is the `Context` object, introduced at Context.

- `local` - the namespace of the current template, described in Built-in Namespaces.

- `self` - the namespace of the topmost template in an inheritance chain (if any, otherwise the same as `local`), mostly described in Inheritance.

- `parent` - the namespace of the parent template in an inheritance chain (otherwise undefined); see Inheritance.

- `next` - the namespace of the next template in an inheritance chain (otherwise undefined); see Inheritance.

- `caller` - a “mini” namespace created when using the `<%call>` tag to define a “def call with content”; described in Calling a Def with Embedded Content and/or Other Defs.

- `loop` - this provides access to `LoopContext` objects when they are requested within `% for` loops, introduced at The Loop Context.

- `capture` - a function that calls a given def and captures its resulting content into a string, which is returned. Usage is described in Filtering and Buffering.

- `UNDEFINED` - a global singleton that is applied to all otherwise uninitialized template variables that were not located within the `Context` when rendering began, unless the `Template` flag `strict_undefined` is set to `True`. `UNDEFINED` is an instance of `Undefined`, and raises an exception when its `__str__()` method is called.

- `pageargs` - this is a dictionary which is present in a template which does not define any `**kwargs` section in its `<%page>` tag. All keyword arguments sent to the `body()` function of a template (when used via namespaces) go here by default unless otherwise defined as a page argument. If this makes no sense, it shouldn’t; read the section The body() Method.

### Reserved Names

Mako has a few names that are considered to be “reserved” and can’t be used as variable names.

Changed in version 0.7: Mako raises an error if these words are found passed to the template as context arguments, whereas in previous versions they’d be silently ignored or lead to other error messages.

- `context` - see Context.

- `UNDEFINED` - see Context Variables.

- `loop` - see The Loop Context. Note this can be disabled for legacy templates via the `enable_loop=False` argument; see Migrating Legacy Templates that Use the Word “loop”.

## API Reference

class `mako.runtime.``Context`(buffer, **data)

  Bases: `object`

  Provides runtime namespace, output buffer, and various callstacks for templates.

  See The Mako Runtime Environment for detail on the usage of `Context`.

  `get`(key, default=None)

    Return a value from this `Context`.

  `keys`()

    Return a list of all names established in this `Context`.

  property `kwargs`

    Return the dictionary of top level keyword arguments associated with this `Context`.

    This dictionary only includes the top-level arguments passed to `Template.render()`. It does not include names produced within the template execution such as local variable names or special names such as `self`, `next`, etc.

    The purpose of this dictionary is primarily for the case that a `Template` accepts arguments via its `<%page>` tag, which are normally expected to be passed via `Template.render()`, except the template is being called in an inheritance context, using the `body()` method. `Context.kwargs` can then be used to propagate these arguments to the inheriting template:

        ${next.body(**context.kwargs)}

  property `lookup`

    Return the `TemplateLookup` associated with this `Context`.

  `pop_caller`()

    Pop a `caller` callable onto the callstack for this `Context`.

  `push_caller`(caller)

    Push a `caller` callable onto the callstack for this `Context`.

  `write`(string)

    Write a string to this `Context` object’s underlying output buffer.

  `writer`()

    Return the current writer function.

class `mako.runtime.``LoopContext`(iterable)

  Bases: `object`

  A magic loop variable. Automatically accessible in any `% for` block.

  See the section The Loop Context for usage notes.

  `parent` -> `LoopContext` or `None`

    The parent loop, if one exists.

  `index` -> int

    The 0-based iteration count.

  `reverse_index` -> int

    The number of iterations remaining.

  `first` -> bool

    `True` on the first iteration, `False` otherwise.

  `last` -> bool

    `True` on the last iteration, `False` otherwise.

  `even` -> bool

    `True` when `index` is even.

  `odd` -> bool

    `True` when `index` is odd.

  `cycle`(*values)

    Cycle through values as the loop progresses.

class `mako.runtime.``Undefined`

  Bases: `object`

  Represents an undefined value in a template.

  All template modules have a constant value `UNDEFINED` present which is an instance of this object.