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Mako 1.3.5 Documentation

Release: 1.3.5
Mako 1.3.5 Documentation » The Mako Runtime Environment

The Mako Runtime Environment

The Mako Runtime Environment

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


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 Template.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 Context.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 Template.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 Template.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.:

  • 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:

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

New in version 0.7.


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 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:

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

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

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

Both approaches produce output like the following:

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

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:

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

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)


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.

API Reference

Object Name Description


Bases: object


Bases: object


Bases: object

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.

method mako.runtime.Context.get(key, default=None)

Return a value from this Context.

method mako.runtime.Context.keys()

Return a list of all names established in this Context.

attribute mako.runtime.Context.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:

attribute mako.runtime.Context.lookup

Return the TemplateLookup associated with this Context.

method mako.runtime.Context.pop_caller()

Pop a caller callable onto the callstack for this Context.

method mako.runtime.Context.push_caller(caller)

Push a caller callable onto the callstack for this Context.

method mako.runtime.Context.write(string)

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

method mako.runtime.Context.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.

method mako.runtime.LoopContext.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.