soapbar 0.5.2

A SOAP framework for Python — client, server, and WSDL handling.

soapbar

A SOAP framework for Python — client, server, and WSDL handling.

soapbar implements SOAP 1.1 and 1.2 with all five binding styles, auto-generates WSDL from Python service classes, parses existing WSDL to drive a typed client, and integrates with any ASGI or WSGI framework via thin adapter classes. The XML parser is hardened against XXE attacks using lxml with resolve_entities=False.

Conformance — soapbar ships with an internal conformance suite of 135 tests across 10 spec-mapped classes (tests/audit/test_compliance.py) covering SOAP 1.1/1.2, WSDL 1.1, and WS-I Basic Profile 1.1. The suite encodes 46 checkpoints derived from F01–F09 original findings, G01–G11 gap findings, I01–I04 informational observations, and S10 (WS-I BSP X.509 token profile); all 46 pass. This is a self-administered test suite, not an independent third-party audit.

---

Table of Contents

1. Features
2. Installation
3. Quick start — server
4. Binding styles and SOAP encoding
5. Defining a service
6. SOAP versions
7. Framework compatibility
8. WSDL
9. Client
10. XSD type system
11. Fault handling
12. Security
13. WS-Security — UsernameToken
14. MTOM/XOP
15. XML Signature and Encryption
16. WSDL schema validation
17. One-way operations
18. SOAP array attributes
19. rpc:result (SOAP 1.2)
20. Interoperability
21. Architecture
22. Public API
23. Comparison with alternatives
24. Development setup
25. Inspired by
26. Learn more
27. Known Limitations
28. License

---

Features

• SOAP 1.1 and 1.2 (auto-detected from envelope namespace; fault codes auto-translated)
• All 5 WSDL/SOAP binding style combinations (RPC/Encoded, RPC/Literal, Document/Literal, Document/Literal/Wrapped, Document/Encoded)
• Auto-generates WSDL from service class definitions — no config files needed
• Parses existing WSDL to drive a typed client
• ASGI adapter (AsgiSoapApp) and WSGI adapter (WsgiSoapApp)
• XXE-safe hardened XML parser (lxml, resolve_entities=False, no_network=True, load_dtd=False)
• Message size limit (10 MB default) and XML nesting depth limit (100 levels) — DoS protection
• WS-Security UsernameToken — PasswordText and PasswordDigest (SHA-1) on both client and server
• XML Signature — enveloped XML-DSIG signing and verification (sign_envelope / verify_envelope, requires signxml)
• XML Encryption — AES-256-CBC body encryption with RSA-OAEP session-key wrapping (encrypt_body / decrypt_body, requires cryptography)
• MTOM/XOP — send and receive SOAP messages with binary attachments; SoapClient(use_mtom=True) + add_attachment(); server decodes inbound MTOM automatically
• WSDL schema validation — opt-in Body validation against WSDL-embedded XSD types (SoapApplication(validate_body_schema=True))
• One-way MEP — @soap_operation(one_way=True) returns HTTP 202 with empty body
• SOAP array attributes — enc:itemType/enc:arraySize (SOAP 1.2) and SOAP-ENC:arrayType (SOAP 1.1) emitted automatically
• Multi-reference encoding — shared complex objects serialized with id/href per SOAP 1.1 §5.2.5
• rpc:result — opt-in @soap_operation(emit_rpc_result=True) per SOAP 1.2 Part 2 §4.2.1
• WS-Addressing 1.0 — MessageID, RelatesTo, Action, ReferenceParameters propagated in responses
• XSD type registry with 27 built-in types
• Sync and async HTTP client (httpx optional)
• Interoperable with zeep and spyne out-of-the-box (verified by integration tests)
• JSON dual-mode — any SOAP endpoint returns JSON when client sends Accept: application/json; no separate endpoint needed
• Non-strict WSDL parsing — parse_wsdl(..., strict=False) silently skips unresolvable imports instead of raising
• Full type annotations + py.typed marker (PEP 561)
• Python 3.10 – 3.14

---

Installation

pip install soapbar              # core + server + WSDL (lxml only)
pip install soapbar[core]        # explicit alias for the above
pip install soapbar[server]      # explicit alias for the above
pip install soapbar[client]      # + httpx for the HTTP client
pip install soapbar[security]    # + signxml + cryptography (XML Sig/Enc)
pip install soapbar[all]         # everything (client + security)

Or with uv:

uv add soapbar
uv add "soapbar[client]"
uv add "soapbar[security]"
uv add "soapbar[all]"

---

Quick start — server

Variant A — standalone (bare ASGI, no framework)

# app.py
from soapbar import SoapService, soap_operation, SoapApplication, AsgiSoapApp


class CalculatorService(SoapService):
    __service_name__ = "Calculator"
    __tns__ = "http://example.com/calculator"

    @soap_operation()
    def add(self, a: int, b: int) -> int:
        return a + b

    @soap_operation()
    def subtract(self, a: int, b: int) -> int:
        return a - b


soap_app = SoapApplication(service_url="http://localhost:8000")
soap_app.register(CalculatorService())

app = AsgiSoapApp(soap_app)
# Run: uvicorn app:app --port 8000
# WSDL: GET http://localhost:8000?wsdl

Variant B — mounted inside FastAPI

from fastapi import FastAPI
from soapbar import SoapApplication, AsgiSoapApp

# ... (same CalculatorService class as above) ...

soap_app = SoapApplication(service_url="http://localhost:8000/soap")
soap_app.register(CalculatorService())

api = FastAPI()
api.mount("/soap", AsgiSoapApp(soap_app))
# Run: uvicorn app:api --port 8000
# WSDL: GET http://localhost:8000/soap?wsdl

---

Binding styles and SOAP encoding

Background — two dimensions

The WSDL <binding> element is described by two orthogonal choices:

• Style: rpc or document — controls whether the SOAP Body contains a wrapper element named after the operation (rpc) or raw parameter elements without a wrapper (document).
• Use: encoded or literal — controls whether each element carries a xsi:type attribute with runtime type information (encoded) or relies solely on the schema (literal).

References:

• IBM developerWorks — Which WSDL style?
• DZone — Different SOAP encoding styles
• Stack Overflow — Document vs RPC style

The five combinations

BindingStyle is importable as from soapbar import BindingStyle.

BindingStyle enum	WSDL style	WSDL use	WS-I BP	Notes
RPC_ENCODED	rpc	encoded	✗	Legacy; params carry xsi:type; operation wrapper in Body
RPC_LITERAL	rpc	literal	✓	No xsi:type; operation wrapper in Body
DOCUMENT_LITERAL	document	literal	✓	Params are direct Body children; no wrapper
DOCUMENT_LITERAL_WRAPPED	document	literal	✓	Default & recommended; single wrapper element named after operation
DOCUMENT_ENCODED	document	encoded	✗	Params are direct Body children each with xsi:type

RPC_ENCODED

<soapenv:Body>
  <tns:add soapenc:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
    <a xsi:type="xsd:int">3</a>
    <b xsi:type="xsd:int">5</b>
  </tns:add>
</soapenv:Body>

RPC_LITERAL

<soapenv:Body>
  <tns:add>
    <a>3</a>
    <b>5</b>
  </tns:add>
</soapenv:Body>

DOCUMENT_LITERAL

<soapenv:Body>
  <a>3</a>
  <b>5</b>
</soapenv:Body>

DOCUMENT_LITERAL_WRAPPED (default)

<soapenv:Body>
  <tns:add>
    <a>3</a>
    <b>5</b>
  </tns:add>
</soapenv:Body>

DOCUMENT_ENCODED

<soapenv:Body>
  <a xsi:type="xsd:int">3</a>
  <b xsi:type="xsd:int">5</b>
</soapenv:Body>

Which to choose?

Use DOCUMENT_LITERAL_WRAPPED unless you are interoperating with a legacy system that requires RPC_ENCODED. DOCUMENT_LITERAL_WRAPPED is WS-I Basic Profile compliant, the most widely supported style, and the easiest to validate with schema tools.

---

Defining a service

from decimal import Decimal
from soapbar import SoapService, soap_operation, BindingStyle, SoapVersion, xsd
from soapbar import OperationParameter


class PricingService(SoapService):
    # Class attributes (all have defaults — only override what you need)
    __service_name__ = "Pricing"
    __tns__ = "http://example.com/pricing"
    __binding_style__ = BindingStyle.DOCUMENT_LITERAL_WRAPPED
    __soap_version__ = SoapVersion.SOAP_11
    __service_url__ = "http://localhost:8000/soap"

    # Auto-introspection: input/output params derived from type hints
    @soap_operation(documentation="Calculate discounted price")
    def get_price(self, item_id: str, quantity: int) -> Decimal:
        return Decimal("9.99") * quantity

    # Explicit params: use when hints are insufficient or unavailable
    @soap_operation(
        input_params=[
            OperationParameter(name="item_id", xsd_type=xsd.resolve("string")),
            OperationParameter(name="quantity", xsd_type=xsd.resolve("int")),
        ],
        output_params=[
            OperationParameter(name="price", xsd_type=xsd.resolve("decimal")),
        ],
    )
    def get_price_explicit(self, item_id: str, quantity: int) -> Decimal:
        return Decimal("9.99") * quantity

SoapService class attribute defaults

Attribute	Default	Notes
__service_name__	class name	Used in WSDL <service name="">
__tns__	"http://example.com/{name}"	Target namespace
__binding_style__	BindingStyle.DOCUMENT_LITERAL_WRAPPED	Recommended default
__soap_version__	SoapVersion.SOAP_11	Change to SOAP_12 if needed
__port_name__	"{name}Port"	WSDL port name
__service_url__	""	Override or pass to SoapApplication

---

SOAP versions

	SOAP 1.1	SOAP 1.2
Envelope namespace	http://schemas.xmlsoap.org/soap/envelope/	http://www.w3.org/2003/05/soap-envelope
Content-Type	text/xml; charset=utf-8	application/soap+xml; charset=utf-8
Action header	SOAPAction: "..." (separate header)	action="..." in Content-Type
Fault code (client)	Client	Sender
Fault code (server)	Server	Receiver

soapbar detects the SOAP version automatically from the envelope namespace and translates fault codes between versions when building responses.

from soapbar import SoapVersion

SoapVersion.SOAP_11   # SOAP 1.1
SoapVersion.SOAP_12   # SOAP 1.2

---

Framework compatibility

ASGI frameworks (via AsgiSoapApp)

AsgiSoapApp is a standard ASGI application. Mount it anywhere an ASGI app is accepted.

Framework	How to mount
FastAPI	app.mount("/soap", AsgiSoapApp(soap_app))
Starlette	routes=[Mount("/soap", app=AsgiSoapApp(soap_app))]
Litestar	app.mount("/soap", AsgiSoapApp(soap_app))
Quart	Use asgiref or serve directly with Hypercorn
BlackSheep	app.mount("/soap", AsgiSoapApp(soap_app))
Django (≥ 3.1 ASGI)	Route in asgi.py via URL dispatcher

ASGI servers (Uvicorn, Hypercorn, Daphne) can run AsgiSoapApp directly.

FastAPI example:

from fastapi import FastAPI
from soapbar import SoapApplication, AsgiSoapApp

soap_app = SoapApplication(service_url="http://localhost:8000/soap")
soap_app.register(CalculatorService())

api = FastAPI()
api.mount("/soap", AsgiSoapApp(soap_app))

WSGI frameworks (via WsgiSoapApp)

Framework	How to mount
Flask	DispatcherMiddleware or replace app.wsgi_app (requires werkzeug)
Django (classic WSGI)	Mount as sub-application in urls.py
Falcon	app.add_sink(WsgiSoapApp(soap_app), "/soap")
Bottle	app.mount("/soap", WsgiSoapApp(soap_app))
Pyramid	Composable WSGI stack

WSGI servers (Gunicorn, uWSGI, mod_wsgi) can run WsgiSoapApp directly.

Flask example:

from flask import Flask
from werkzeug.middleware.dispatcher import DispatcherMiddleware
from soapbar import SoapApplication, WsgiSoapApp

soap_app = SoapApplication(service_url="http://localhost:8000/soap")
soap_app.register(CalculatorService())

flask_app = Flask(__name__)
flask_app.wsgi_app = DispatcherMiddleware(flask_app.wsgi_app, {
    "/soap": WsgiSoapApp(soap_app),
})

---

WSDL

Auto-generation — no configuration needed. Register a service and the WSDL is generated automatically:

wsdl_bytes = soap_app.get_wsdl()

Served automatically at GET ?wsdl when using AsgiSoapApp or WsgiSoapApp.

Parse an existing WSDL to inspect its structure:

from soapbar import parse_wsdl, parse_wsdl_file

defn = parse_wsdl(wsdl_bytes)          # from bytes/str
defn = parse_wsdl_file("service.wsdl") # from file

Custom WSDL override — supply your own WSDL document and skip auto-generation:

soap_app = SoapApplication(custom_wsdl=open("my_service.wsdl", "rb").read())

Remote wsdl:import — SSRF guard — parse_wsdl blocks outbound HTTP fetches by default. wsdl:import elements whose resolved location starts with http:// or https:// raise ValueError unless you explicitly opt in:

# Default — safe for untrusted WSDLs; remote imports raise ValueError
defn = parse_wsdl(wsdl_bytes)

# Opt-in — only when the WSDL source is trusted
defn = parse_wsdl(wsdl_bytes, allow_remote_imports=True)

This prevents Server-Side Request Forgery (SSRF) when parsing WSDLs from user-supplied URLs or untrusted data. The top-level WSDL fetch (e.g. SoapClient(wsdl_url=...)) is always explicit; only wsdl:import resolution inside the document is guarded.

---

Client

import asyncio
from soapbar import SoapClient, SoapFault

# From a live WSDL URL (fetches WSDL over HTTP)
client = SoapClient(wsdl_url="http://localhost:8000/soap?wsdl")

# From a WSDL string/bytes you already have
client = SoapClient.from_wsdl_string(wsdl_bytes)

# From a WSDL file
client = SoapClient.from_file("service.wsdl")

# Manual — no WSDL, specify endpoint and style directly
from soapbar import BindingStyle, SoapVersion

client = SoapClient.manual(
    address="http://localhost:8000/soap",
    binding_style=BindingStyle.DOCUMENT_LITERAL_WRAPPED,
    soap_version=SoapVersion.SOAP_11,
)

# Sync call via service proxy
try:
    result = client.service.add(a=3, b=5)
    print(result)  # 8
except SoapFault as fault:
    print(fault.faultcode, fault.faultstring)

# Direct call by operation name
result = client.call("add", a=3, b=5)

# Async call
async def main():
    result = await client.call_async("add", a=3, b=5)
    print(result)

asyncio.run(main())

HttpTransport options

from soapbar import SoapClient, HttpTransport

transport = HttpTransport(timeout=60.0, verify_ssl=False)
client = SoapClient(wsdl_url="http://localhost:8000/soap?wsdl", transport=transport)

Advanced: manual client with explicit operation signature

Use register_operation when you need full control over the operation schema without a WSDL:

from soapbar import SoapClient, OperationSignature, OperationParameter, BindingStyle, xsd

sig = OperationSignature(
    name="Add",
    input_params=[
        OperationParameter("a", xsd.resolve("int")),
        OperationParameter("b", xsd.resolve("int")),
    ],
    output_params=[OperationParameter("return", xsd.resolve("int"))],
)

client = SoapClient.manual("http://host/soap", binding_style=BindingStyle.RPC_LITERAL)
client.register_operation(sig)
result = client.call("Add", a=3, b=4)  # 7

---

XSD type system

soapbar includes a registry of 27 built-in XSD types. Types handle serialization to and from XML text.

from soapbar import xsd

# Resolve a type by XSD name
int_type = xsd.resolve("int")        # XsdType for xsd:int
str_type = xsd.resolve("string")     # XsdType for xsd:string

# Map a Python type to its XSD equivalent
xsd_type = xsd.python_to_xsd(int)    # -> xsd:int XsdType
xsd_type = xsd.python_to_xsd(str)    # -> xsd:string XsdType

# Serialize / deserialize
int_type.to_xml(42)       # "42"
int_type.from_xml("42")   # 42

# Inspect all registered types
all_types = xsd.all_types()

Python → XSD mapping:

Python type	XSD type
bool	boolean
int	int
float	float
str	string
Decimal	decimal
bytes	base64Binary

---

Fault handling

Raising a fault from a service method

from soapbar import SoapService, soap_operation, SoapFault


class StrictCalculator(SoapService):
    __service_name__ = "StrictCalculator"
    __tns__ = "http://example.com/calc"

    @soap_operation()
    def divide(self, a: int, b: int) -> int:
        if b == 0:
            raise SoapFault(
                faultcode="Client",
                faultstring="Division by zero",
                detail="b must be non-zero",
            )
        return a // b

SoapClient.call() and call_async() automatically raise SoapFault when the server returns a fault response.

Creating and rendering faults manually

from soapbar import SoapFault

# Create a fault
fault = SoapFault(
    faultcode="Client",
    faultstring="Invalid input: quantity must be positive",
    detail="quantity=-1",          # string or lxml _Element
)

# Render as SOAP 1.1 or 1.2 envelope
envelope_11 = fault.to_soap11_envelope()
envelope_12 = fault.to_soap12_envelope()

# SOAP 1.2 subcodes — each is (namespace_uri, localname) for spec-compliant QName
fault_12 = SoapFault(
    faultcode="Client",
    faultstring="Validation error",
    subcodes=[("http://example.com/errors", "InvalidQuantity")],
)

Fault code translation is automatic:

Canonical (used in soapbar)	SOAP 1.1 wire	SOAP 1.2 wire
Client	Client	Sender
Server	Server	Receiver

---

Security

soapbar uses a hardened lxml parser:

lxml.etree.XMLParser(
    resolve_entities=False,   # XXE prevention
    no_network=True,          # SSRF prevention
    load_dtd=False,           # DTD injection prevention
    huge_tree=False,          # Billion-Laughs prevention
    remove_comments=True,     # comment injection prevention
    remove_pis=True,
)

Entity references (potential XXE payloads) are silently dropped rather than expanded. No network connections are made during parsing. DTDs are not loaded.

Additional hardening:

• Message size limit: SoapApplication(max_body_size=10*1024*1024) — requests exceeding 10 MB are rejected with a Client fault before XML parsing.
• XML nesting depth: requests exceeding 100 levels of nesting are rejected to prevent stack exhaustion.
• Error scrubbing: unhandled exceptions produce "An internal error occurred." — no stack traces or exception text are returned to clients.
• HTTPS warning: SoapApplication warns at construction time if service_url uses plain HTTP.

---

WS-Security — UsernameToken

soapbar supports WS-Security 1.0 UsernameToken (OASIS 2004), both plain-text and SHA-1 digest.

Client — attaching credentials

from soapbar import SoapClient
from soapbar.core.wssecurity import UsernameTokenCredential

# Plain-text password
cred = UsernameTokenCredential(username="alice", password="secret")

# SHA-1 PasswordDigest (recommended for non-TLS scenarios)
cred = UsernameTokenCredential(username="alice", password="secret", use_digest=True)

client = SoapClient.manual(
    "https://example.com/soap",
    wss_credential=cred,
)
result = client.call("GetData", id=42)

The wsse:Security header is injected automatically on every call.

Server — validating credentials

from soapbar import SoapApplication
from soapbar.core.wssecurity import UsernameTokenValidator, SecurityValidationError


class MyValidator(UsernameTokenValidator):
    _users = {"alice": "secret", "bob": "hunter2"}

    def get_password(self, username: str) -> str | None:
        return self._users.get(username)


app = SoapApplication(
    service_url="https://example.com/soap",
    security_validator=MyValidator(),
)
app.register(MyService())

SecurityValidationError is converted to a Client SOAP fault automatically. Both PasswordText and PasswordDigest token types are verified; Digest requires wsse:Nonce and wsu:Created to be present.

---

MTOM/XOP

soapbar supports MTOM (Message Transmission Optimization Mechanism, W3C) for sending and receiving SOAP messages with binary attachments. The multipart/related MIME packaging is handled transparently — the core envelope sees resolved base64 data; your service code sees plain bytes.

Client — sending attachments

from soapbar import SoapClient, BindingStyle

client = SoapClient.manual(
    "http://localhost:8000/soap",
    binding_style=BindingStyle.DOCUMENT_LITERAL_WRAPPED,
    use_mtom=True,
)

# Queue a binary attachment and get its Content-ID back
cid = client.add_attachment(b"\x89PNG...", content_type="image/png")

# The call packages the envelope + attachments as multipart/related
result = client.call("UploadImage", image_cid=cid, filename="logo.png")

Server — receiving MTOM

No configuration required. AsgiSoapApp and WsgiSoapApp automatically detect inbound multipart/related requests, resolve all xop:Include references inline, and pass the reconstructed XML to the dispatcher as a normal SOAP envelope.

Low-level API

from soapbar import parse_mtom, build_mtom, MtomAttachment

# Parse a raw MTOM HTTP body
msg = parse_mtom(raw_bytes, content_type_header)
print(msg.soap_xml)       # bytes — envelope with XOP includes resolved
print(msg.attachments)    # list[MtomAttachment]

# Build a MTOM HTTP body
attachments = [MtomAttachment(content_id="part1@host", content_type="image/png", data=png_bytes)]
body_bytes, content_type = build_mtom(soap_xml_bytes, attachments)

---

XML Signature and Encryption

Requires pip install soapbar[security] (pulls in signxml and cryptography).

XML Digital Signature (XML-DSIG)

from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.hazmat.primitives import hashes
from cryptography.x509 import CertificateBuilder
from soapbar.core.wssecurity import sign_envelope, verify_envelope, XmlSecurityError

# Sign — enveloped RSA-SHA256 XML-DSIG
signed_bytes = sign_envelope(envelope_bytes, private_key, certificate)

# Verify — raises XmlSecurityError on bad signature
try:
    verified_bytes = verify_envelope(signed_bytes, certificate)
except XmlSecurityError as exc:
    print("Signature invalid:", exc)

XML Encryption (AES-256-CBC + RSA-OAEP)

from soapbar.core.wssecurity import encrypt_body, decrypt_body, XmlSecurityError

# Encrypt SOAP Body — AES-256-CBC session key wrapped with recipient's RSA public key
encrypted_bytes = encrypt_body(envelope_bytes, recipient_public_key)

# Decrypt — extracts and unwraps the session key, restores Body children
decrypted_bytes = decrypt_body(encrypted_bytes, recipient_private_key)

The xenc:EncryptedData element is placed as the sole child of <soap:Body>. The AES-256-CBC session key is wrapped with RSA-OAEP (SHA-256) in an xenc:EncryptedKey element inside xenc:KeyInfo.

WS-I BSP X.509 Token Profile (S10)

For interoperability with WS-I Basic Security Profile 1.1 compliant clients and servers, use the BSP variant which embeds the certificate as a wsse:BinarySecurityToken and references it from ds:Signature/ds:KeyInfo:

from soapbar.core.wssecurity import (
    sign_envelope_bsp,
    verify_envelope_bsp,
    build_binary_security_token,
    extract_certificate_from_security,
)

# Sign — adds wsse:BinarySecurityToken + wsse:SecurityTokenReference in KeyInfo
signed_bytes = sign_envelope_bsp(envelope_bytes, private_key, certificate)

# Verify — extracts cert from BST, verifies ds:Signature
verified_bytes = verify_envelope_bsp(signed_bytes)

# Build a standalone BinarySecurityToken element (e.g. to add to an existing header)
bst = build_binary_security_token(certificate, token_id="MyToken-1")

---

WSDL schema validation

SoapApplication can validate the SOAP Body of each inbound request against the XSD types embedded in the WSDL. Validation is opt-in and disabled by default.

from soapbar import SoapApplication

soap_app = SoapApplication(
    service_url="https://example.com/soap",
    validate_body_schema=True,   # X07 — WS-I BP 1.1 R2201
)
soap_app.register(MyService())

When enabled, the compiled lxml.etree.XMLSchema is built once from the WSDL-embedded <xs:schema> elements and cached. Any Body element that fails schema validation results in a Client fault with the first schema error message. Requests to services with no embedded schemas pass through unchanged.

---

One-way operations

One-way operations fire-and-forget: the server processes the message and returns HTTP 202 Accepted with an empty body (SOAP 1.2 Part 2 §7.5.1).

from soapbar import SoapService, soap_operation


class EventService(SoapService):
    __service_name__ = "EventService"
    __tns__ = "http://example.com/events"

    @soap_operation(one_way=True)
    def publish_event(self, event_type: str, payload: str) -> None:
        # Process asynchronously — no response is sent
        _event_queue.put((event_type, payload))

The client receives 202 Accepted with no body. SoapClient.call() returns None for one-way operations.

---

SOAP array attributes

When using encoded binding styles (RPC_ENCODED, DOCUMENT_ENCODED), array elements are annotated with the correct version-specific attributes automatically.

SOAP 1.1 (SOAP-ENC:arrayType):

<names soapenc:arrayType="xsd:string[3]"
       xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/">
  <item>Alice</item><item>Bob</item><item>Carol</item>
</names>

SOAP 1.2 (enc:itemType + enc:arraySize):

<names enc:itemType="xsd:string" enc:arraySize="3"
       xmlns:enc="http://www.w3.org/2003/05/soap-encoding">
  <item>Alice</item><item>Bob</item><item>Carol</item>
</names>

The correct attributes are emitted automatically based on the SOAP version in use — no manual configuration needed. The get_serializer(style, soap_version) factory handles the selection.

---

rpc:result (SOAP 1.2)

SOAP 1.2 Part 2 §4.2.1 defines a rpc:result SHOULD convention for naming the return value in RPC responses. soapbar omits it by default (preserving interoperability with zeep and other strict-mode clients) and offers an opt-in:

from soapbar import SoapService, soap_operation


class CalcService(SoapService):
    __service_name__ = "Calc"
    __tns__ = "http://example.com/calc"

    # Default: no rpc:result (interoperable with zeep, WCF, etc.)
    @soap_operation()
    def add(self, a: int, b: int) -> int:
        return a + b

    # Opt-in: emit rpc:result for strict SOAP 1.2 consumers
    @soap_operation(emit_rpc_result=True)
    def add_strict(self, a: int, b: int) -> int:
        return a + b

When opted in, the response wrapper contains:

<CalcResponse>
  <rpc:result xmlns:rpc="http://www.w3.org/2003/05/soap-rpc">return</rpc:result>
  <return>8</return>
</CalcResponse>

---

Interoperability

soapbar is tested against zeep and spyne via integration tests.

• zeep → soapbar: a zeep client can call a soapbar server without modification. The WSDL generated by soapbar is zeep-parseable.
• soapbar → spyne: a soapbar client can call a spyne server using RPC/Literal.
• soapbar ↔ soapbar: full round-trip tested for all binding styles and both SOAP versions.

---

Architecture

  HTTP request
       │
       ▼
┌─────────────────┐
│ AsgiSoapApp /   │   ← thin ASGI/WSGI adapters
│ WsgiSoapApp     │
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│ SoapApplication │   ← dispatcher: version detection,
│                 │     operation routing, fault wrapping
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│  SoapService    │   ← your business logic lives here
│  @soap_operation│
└────────┬────────┘
         │ calls binding serializer + envelope builder
         ▼
┌─────────────────┐
│  core/          │   ← binding.py · envelope.py · types.py
│  binding/types/ │       wsdl/ · xml.py · fault.py
│  envelope/wsdl  │
└─────────────────┘

---

Public API

The most-used symbols are all importable from the top-level soapbar namespace:

Symbol	Import	Description
SoapService	from soapbar import SoapService	Base class for SOAP services
soap_operation	from soapbar import soap_operation	Decorator for service methods
SoapApplication	from soapbar import SoapApplication	SOAP dispatcher/router
AsgiSoapApp	from soapbar import AsgiSoapApp	ASGI adapter
WsgiSoapApp	from soapbar import WsgiSoapApp	WSGI adapter
SoapClient	from soapbar import SoapClient	SOAP client
HttpTransport	from soapbar import HttpTransport	HTTP transport layer
SoapFault	from soapbar import SoapFault	SOAP fault exception
BindingStyle	from soapbar import BindingStyle	Binding style enum
SoapVersion	from soapbar import SoapVersion	SOAP version enum
xsd	from soapbar import xsd	XSD type registry
parse_wsdl	from soapbar import parse_wsdl	Parse WSDL from bytes/str
parse_wsdl_file	from soapbar import parse_wsdl_file	Parse WSDL from a file path
build_wsdl_string	from soapbar import build_wsdl_string	Generate WSDL as string
OperationParameter	from soapbar import OperationParameter	Parameter descriptor for operations
OperationSignature	from soapbar import OperationSignature	Full operation signature (manual client)
UsernameTokenCredential	from soapbar.core.wssecurity import UsernameTokenCredential	WS-Security credential for client
UsernameTokenValidator	from soapbar.core.wssecurity import UsernameTokenValidator	Abstract base for server-side token validation
SecurityValidationError	from soapbar.core.wssecurity import SecurityValidationError	Raised on authentication failure
build_security_header	from soapbar.core.wssecurity import build_security_header	Build wsse:Security header element
sign_envelope	from soapbar.core.wssecurity import sign_envelope	Enveloped XML-DSIG signature (RSA-SHA256)
verify_envelope	from soapbar.core.wssecurity import verify_envelope	Verify and return signed envelope bytes
encrypt_body	from soapbar.core.wssecurity import encrypt_body	AES-256-CBC body encryption + RSA-OAEP key wrap
decrypt_body	from soapbar.core.wssecurity import decrypt_body	Decrypt xenc:EncryptedData body and restore children
XmlSecurityError	from soapbar.core.wssecurity import XmlSecurityError	Raised on XML signature/encryption failure
build_binary_security_token	from soapbar.core.wssecurity import build_binary_security_token	Build WS-I BSP wsse:BinarySecurityToken from X.509 cert
extract_certificate_from_security	from soapbar.core.wssecurity import extract_certificate_from_security	Extract X.509 cert from wsse:BinarySecurityToken
sign_envelope_bsp	from soapbar.core.wssecurity import sign_envelope_bsp	BSP-compliant signing with wsse:SecurityTokenReference
verify_envelope_bsp	from soapbar.core.wssecurity import verify_envelope_bsp	Verify BSP-signed envelope using embedded BST cert
MtomAttachment	from soapbar import MtomAttachment	MTOM attachment descriptor (content_id, content_type, data)
MtomMessage	from soapbar import MtomMessage	Parsed MTOM message (soap_xml + attachments list)
parse_mtom	from soapbar import parse_mtom	Parse a raw multipart/related MTOM body
build_mtom	from soapbar import build_mtom	Build a multipart/related MTOM body

---

Comparison with alternatives

Capability	soapbar	zeep	spyne	fastapi-soap
SOAP client	✓	✓	✗	✗
SOAP server	✓	✗	✓	✓
All 5 binding styles	✓	✓ (client)	✓	Partial
SOAP 1.1 + 1.2	✓	✓	✓	1.1 only
ASGI frameworks	✓	✗	✗	FastAPI only
WSGI frameworks	✓	✗	✓	✗
Auto WSDL generation	✓	✗	✓	✓
WSDL-driven client	✓	✓	✗	✗
XXE hardened by default	✓	undocumented	undocumented	undocumented
Message size + depth limits	✓	✗	✗	✗
WS-Security UsernameToken	✓	✓ (client)	✓	✗
XML Signature / Encryption	✓ ([security])	✗	Partial	✗
MTOM/XOP	✓	✓	✓	✗
WS-Addressing 1.0	✓	✓	Partial	✗
One-way MEP (HTTP 202)	✓	✓	✓	✗
SOAP array attributes	✓	✓	✓	✗
Internal conformance suite (135 tests)	✓	not claimed	not claimed	not claimed
Core dependency	lxml	lxml, requests	lxml	fastapi, lxml
Async HTTP client	httpx (optional)	httpx (optional)	—	—
Python versions	3.10–3.14	3.8+	3.8+	3.8+

soapbar is the only Python library that covers both client and server, works with any ASGI or WSGI framework, supports SOAP 1.1 and 1.2, is hardened against XXE/DoS attacks out of the box, and ships with an internal conformance suite of 135 tests covering 46 spec-derived checkpoints (see tests/audit/test_compliance.py).

---

Development setup

git clone https://github.com/hitoshyamamoto/soapbar
cd soapbar
uv sync --group dev --group lint --group type

# Run tests
uv run pytest tests/ -v

# Lint
uv run ruff check src/ tests/

# Type check
uv run mypy src/

Run the example server (requires FastAPI + uvicorn):

pip install fastapi uvicorn
uvicorn examples.calculator_fastapi:app --reload --port 8000

Then fetch the WSDL: curl http://localhost:8000/soap?wsdl

---

Inspired by

• Spyne — the original comprehensive Python SOAP/RPC framework; inspired the service-class model and binding style abstractions.
• zeep — the de facto modern Python SOAP client; inspired the WSDL-driven client approach and XSD type mapping.
• fastapi-soap — demonstrated clean FastAPI/ASGI integration for SOAP endpoints; inspired the ASGI adapter design.

---

Learn more

SOAP protocol

• Wikipedia — SOAP
• W3Schools — XML/SOAP intro
• GeeksForGeeks — Basics of SOAP
• Oracle — SOAP API reference

WSDL

• TutorialsPoint — WSDL
• GeeksForGeeks — WSDL introduction

Binding styles and encoding

• IBM developerWorks — Which WSDL style?
• DZone — Different SOAP encoding styles
• Stack Overflow — Document vs RPC style

---

Known Limitations

The following features are intentionally out-of-scope for the current release. Behaviour is well-defined in each case (documented exception or graceful exposure).

Area	Status	Notes
MTOM/XOP	Fully implemented	parse_mtom / build_mtom handle multipart/related MIME packaging and XOP Include resolution. AsgiSoapApp and WsgiSoapApp decode inbound MTOM automatically. SoapClient sends MTOM when use_mtom=True.
WS-Security	Fully implemented	UsernameTokenCredential / UsernameTokenValidator for PasswordText and PasswordDigest. sign_envelope / verify_envelope for XML-DSIG. encrypt_body / decrypt_body for XML Encryption (AES-256-CBC + RSA-OAEP). sign_envelope_bsp / verify_envelope_bsp + build_binary_security_token for WS-I BSP X.509 token profile (S10). All require soapbar[security].
WS-Addressing	Fully parsed + response headers generated	Inbound headers (MessageID, To, Action, ReplyTo, FaultTo, ReferenceParameters) are parsed into WsaHeaders. Response headers (MessageID, RelatesTo, Action, ReferenceParameters) are generated automatically when use_wsa=True.
SOAP 1.2 relay attribute	Parsed and exposed on SoapHeaderBlock	The relay boolean is available on each SoapHeaderBlock instance. Full SOAP intermediary forwarding (actually relaying the message) is not implemented.
xsd:complexType / xsd:array / xsd:choice	Fully supported for round-trip serialization	Recursive (self-referencing) complex types are resolved lazily. xsd:complexContent/restriction for SOAP-encoded arrays is also parsed from WSDL.
External schema xsd:import	Not followed	wsdl:import (document-level) is resolved with an SSRF guard (allow_remote_imports=False by default). xsd:import elements inside a <types> schema are silently ignored; type resolution falls back to built-in primitives.

---

License

Apache License 2.0 — see LICENSE and NOTICE.