# Python Web Service API¶

The web service API is a collection of Python functions meant to help write web service scripts in as “Pythonic” a way as possible. The functionality is divided into severul areas: core services for generally useful utilities; HTTP interaction, for manipulating request headers, retrieving request bodies, and formatting errors; and web service utilities to supercharge Python services.

## Core Services¶

tangelo.log([context, ]msg[, lvl=loglevel])

Writes a message msg to the log file. The optional context is a descriptive tag that will be prepended to the message within the log file (defaulting to “TANGELO” if omitted). Common context tags used internally in Tangelo include “TANGELO” (to describe startup/shutdown activities), and “ENGINE” (which describes actions being taken by CherryPy). This function may be useful for debugging or otherwise tracking a service’s activities as it runs. The optional logging level lvl is one of the python logging constants. By default, logging.INFO is used.

Generally you should use one of the variants of this function listed below, but if you want to write a logging message in the terminal’s default color, you can use this function, specifying the log level you need.

tangelo.log_debug([context, ]msg)

Variant of tangelo.log() that writes out messages in blue, at level logging.DEBUG. These messages can be used to diagnose, e.g., plugins or services in development. By default, these messages are hidden - you can increase the verbosity to see them.

tangelo.log_info([context, ]msg)

Variant of tangelo.log() that writes out messages in purple at level logging.INFO. Informational messages describe what the system is doing at the moment. For example, when a plugin is about to perform initialization, a call like tangelo.log_info("FOOBAR", "About to initialize...") may be appropriate.

tangelo.log_warning([context, ]msg)

Variant of tangelo.log() that writes out messages in yellow at level logging.WARNING. Warnings are messages indicating that something did not work out as expected, but the requested action will still be accomplished (perhaps differently than was expected by the user). For example, if a plugin needs to fall back on a default method for doing something because the requested method was not available, this might be reported by a warning message.

tangelo.log_error([context, ]msg)

Variant of tangelo.log() that writes out messages in red at level logging.ERROR. Errors describe instances when Tangelo or a plugin fails to perform a requested task, without compromising Tangelo’s ability to continue running. For example, if a plugin requires a file that is missing, it might report an error condition using this function.

tangelo.log_critical([context, ]msg)

Variant of tangelo.log() that writes out messages in bright, bold red at level logging.CRITICAL. Critical errors describe conditions that immediately prevent the further functioning of the system, generally leading to Tangelo halting. Generally, you will not need to call this function.

## HTTP Interaction¶

tangelo.content_type([type])

Returns the content type for the current request, as a string. If type is specified, also sets the content type to the specified string.

tangelo.http_status(code[, message])

Sets the HTTP status code for the current request’s response. code should be an integer; optional message can give a concise description of the code. Omitting it results in a standard message; for instance, tangelo.http_status(404) will send back a status of 404 Not Found.

This function can be called before returning, e.g., a dict describing in detail what went wrong. Then, the response will indicate the general error while the body contains error details, which may be informational for the client, or useful for debugging.

tangelo.header(header_name[, new_value])

Returns the value associated to header_name in the HTTP headers, or None if the header is not present.

If new_value is supplied, the header value will additionally be replaced by that value.

tangelo.request_header(header_name)

Returns the value associated to header_name in the request headers, or None if the header is not present.

tangelo.request_path()

Returns the path of the current request. This is generally the sequence of path components following the domain and port number in a URL.

tangelo.request_body()

Returns a filelike object that streams out the body of the current request. This can be useful, e.g., for retrieving data submitted in the body for a POST request.

tangelo.session(key[, value])

Returns the value currently associated to the session key key, or None if there is no such key. If value is given, it will become newly associated to key.

tangelo.redirect(path[, status_code])

Used to signal the browser that a web service wants to perform an HTTP redirect to a different path. The optional status_code should be a value in the 3xx range indicating the type of redirect desired; it defaults to 303.

In the following example service,

import tangelo

def run():
return tangelo.redirect("other/path/content.html")


Tangelo will direct the client to the URL shown, resulting in that file being served instead of the service itself.

tangelo.internal_redirect(path)

Used to signal the server to serve content from a different path in place of the current service; similar to tangelo.redirect() but without informing the client of the redirection.

The example above will look very similar using this function instead:

import tangelo

def run():
return tangelo.internal_redirect("other/path/content.html")


When this internal redirection occurs, the browser’s displayed URL, for example, will not change to reflect the requested path.

tangelo.file(path[, content_type="application/octet-stream"])

Used to signal the server to serve content from the file at path as if it were being served statically. By using appropriate absolute or relative paths, this function can be used to serve content from a Tangelo service that is not otherwise available from Tangelo’s web root. It can be used as follows:

import tangelo

def run():
return tangelo.file("/some/crazy/path/to/content.txt", content_type="text/plain")


## Web Services Utilities¶

tangelo.paths(paths)

Augments the Python system path with the list of web directories specified in paths. Each path must be within the web root directory or within a user’s web home directory (i.e., the paths must be legal with respect to tangelo.legal_path()).

This function can be used to let web services access commonly used functions that are implemented in their own Python modules somewhere in the web filesystem.

After a service calling this function returns, the system path will be restored to its original state. This requires calling tangelo.paths() in every function wishing to change the path, but prevents shadowing of expected locations by modules with the same name in other directories, and the uncontrolled growth of the sys.path variable.

tangelo.config()

Returns a copy of the service configuration dictionary (see Configuring Web Services).

@tangelo.restful

Marks a function in a Python service file as being part of that service’s RESTful API. This prevents accidental exposure of unmarked support functions as part of the API, and also enables the use of arbitrary words as REST verbs (so long as those words are also valid Python function names). An example usage might look like the following, which uses a both a standard verb (“GET”) and a custom one (“NORMALIZE”).

import tangelo

@tangelo.restful
def get(foo, bar, baz=None):
pass

@tangelo.restful
def normalize():
pass


Note that Tangelo automatically converts the verb used by the web client to all lowercase letters before searching the Python module for a matching function to call.

@tangelo.types(arg1=type1, ..., argN=typeN)

Decorates a service by converting it from a function of several string arguments to a function taking typed arguments. Each argument to tangelo.types() is a function that converts strings to some other type - the standard Python functions int(), float(), and json.loads() are good examples. The functions are passed in as keyword arguments, with the keyword naming an argument in the decorated function. For example, the following code snippet

import tangelo

def stringfunc(a, b):
return a + b

@tangelo.types(a=int, b=int)
def intfunc(a, b):
return a + b

print stringfunc("3", "4")
print intfunc("3", "4")


will print:

34
7


stringfunc() performs string concatentation, while intfunc() performs addition on strings that have been converted to integers.

Though the names of the built-in conversion functions make this decorator look like it accepts “types” as arguments, any function that maps strings to any type can be used. For instance, a string representing the current time could be consumed by a function that parses the string and returns a Python datetime object, or, as mentioned above, json.loads() could be used to convert arbitrary JSON data into Python objects.

If an exception is raised by any of the conversion functions, its error message will be passed back to the client via a tangelo.HTTPStatusCode object.

@tangelo.return_type(type)

Similarly to how tangelo.types() works, this decorator can be used to provide a function to convert the return value of a service function to some other type or form. By default, return values are converted to JSON via the standard json.dumps() function. However, this may not be sufficient in certain cases. For example, the bson.dumps() is a function provided by PyMongo that can handle certain types of objects that json.dumps() cannot, such as datetime objects. In such a case, the service module can provide whatever functions it needs (e.g., by importing an appropriate module or package) then naming the conversion function in this decorator.