Python error handling patterns including input validation, exception hierarchies, and partial failure handling. Use when implementing validation logic, designing exception strategies, handling batch processing failures, or building robust APIs.
Build robust Python applications with proper input validation, meaningful exceptions, and graceful failure handling. Good error handling makes debugging easier and systems more reliable.
Validate inputs early, before expensive operations. Report all validation errors at once when possible.
Use appropriate exception types with context. Messages should explain what failed, why, and how to fix it.
In batch operations, don't let one failure abort everything. Track successes and failures separately.
Chain exceptions to maintain the full error trail for debugging.
def fetch_page(url: str, page_size: int) -> Page:
if not url:
raise ValueError("'url' is required")
if not 1 <= page_size <= 100:
raise ValueError(f"'page_size' must be 1-100, got {page_size}")
# Now safe to proceed...
Validate all inputs at API boundaries before any processing begins.
def process_order(
order_id: str,
quantity: int,
discount_percent: float,
) -> OrderResult:
"""Process an order with validation."""
# Validate required fields
if not order_id:
raise ValueError("'order_id' is required")
# Validate ranges
if quantity <= 0:
raise ValueError(f"'quantity' must be positive, got {quantity}")
if not 0 <= discount_percent <= 100:
raise ValueError(
f"'discount_percent' must be 0-100, got {discount_percent}"
)
# Validation passed, proceed with processing
return _process_validated_order(order_id, quantity, discount_percent)
Parse strings and external data into typed domain objects at system boundaries.
from enum import Enum
class OutputFormat(Enum):
JSON = "json"
CSV = "csv"
PARQUET = "parquet"
def parse_output_format(value: str) -> OutputFormat:
"""Parse string to OutputFormat enum.
Args:
value: Format string from user input.
Returns:
Validated OutputFormat enum member.
Raises:
ValueError: If format is not recognized.
"""
try:
return OutputFormat(value.lower())
except ValueError:
valid_formats = [f.value for f in OutputFormat]
raise ValueError(
f"Invalid format '{value}'. "
f"Valid options: {', '.join(valid_formats)}"
)
# Usage at API boundary
def export_data(data: list[dict], format_str: str) -> bytes:
output_format = parse_output_format(format_str) # Fail fast
# Rest of function uses typed OutputFormat
...
Use Pydantic models for structured input validation with automatic error messages.
from pydantic import BaseModel, Field, field_validator
class CreateUserInput(BaseModel):
"""Input model for user creation."""
email: str = Field(..., min_length=5, max_length=255)
name: str = Field(..., min_length=1, max_length=100)
age: int = Field(ge=0, le=150)
@field_validator("email")
@classmethod
def validate_email_format(cls, v: str) -> str:
if "@" not in v or "." not in v.split("@")[-1]:
raise ValueError("Invalid email format")
return v.lower()
@field_validator("name")
@classmethod
def normalize_name(cls, v: str) -> str:
return v.strip().title()
# Usage