The single responsibility principle

The single responsibility principle

The single responsibility principle (SRP) instructs developers to write code that has one and only one reason to change. If a class has more than one reason to change, it has more than one responsibility.

The single responsibility principle (SRP) instructs developers to write code that has one and only one reason to change. If a class has more than one reason to change, it has more than one responsibility. Classes with more than a single responsibility should be broken down into smaller classes, each of which should have only one responsibility and reason to change.

This article explains that process and shows you how to create classes that have only a single responsibility but are still useful. Through a process of delegation and abstraction, a class that contains too many reasons to change should delegate one or more responsibilities to other classes.

It is difficult to overstate the importance of delegating to abstractions. It is the lynchpin of adaptive code and, without it, developers would struggle to adapt to changing requirements in the way that Scrum, Kanban, and other Agile frameworks demand.

Problem statement

To better explain the problem with having classes that hold too many responsibilities, this section explores an example. Listing 1 shows the TradeProcessor class. This class reads records from a file and updates a database.

Despite its small size, this sort of code is common and often needs to cope with new features and changes to requirements

class TradeProcessor:
  lots_size: float = 100000.0

  def process_trades(self, stream: Sequence[str]) -> None:
    lines: List[str] = []
    line: str
    for line in stream:
    trades: List[TradeRecord] = []
    line_count: int = 1
    for line in lines:
      fields: List[str] = line.split(',')
      if len(fields) != 3:
        print(f'WARN: Line {line_count} malformed. Only {len(fields)} fields(s) found.')
      if len(fields[0]) !=6:
        print(f"WARN: Trade currencies on line {line_count} malformed: '{len(fields)}'")
      trade_amount: Optional[int] = int_try_parse(fields[1])
      if trade_amount is None:
        print(f"WARN: Trade amount on line {line_count} not a valid integer: '{fields[1]}'")
      trade_price: Optional[float] = float_try_parse(fields[2])
      if trade_price is None:
        print(f"WARN: Trade price on line {line_count} not a valid decimal: '{fields[2]}'")
      source_currency_code: str = fields[0][:3]
      destination_currency_code: str = fields[0][3:6]

      trade = TradeRecord(source_currency_code,
      line_count += 1

    engine = create_engine('sqlite:///trades.db', echo=False)
    metadata = MetaData(engine)
    tbl = Table('trade_table',
    for trade in trades:
      ins = tbl.insert(None).values(
            conn = engine.connect()
print(f'INFO: {len(trades)} trades processed')

refactoring interface-design agile-methodology refactor python

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