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Generator expressions

Generator expressions let you process values lazily instead of building a full list first. They are especially useful when you only need to iterate once or pass values directly into another function.

Why this matters​

This pattern builds an unnecessary list:

total = sum([value * 2 for value in numbers])

It works, but the intermediate list is only used once.

Prefer a generator expression​

total = sum(value * 2 for value in numbers)

This version streams values into sum() without creating a separate list in memory first.

That is often:

  • more memory efficient
  • just as readable
  • a better fit for one-pass operations

Where generator expressions shine​

They work especially well with functions like:

  • sum()
  • any()
  • all()
  • max()
  • min()

For example:

has_error = any("ERROR" in line for line in lines)

The values are produced only as needed.

When a list is still the right choice​

Use a list comprehension instead when:

  • you need to reuse the result multiple times
  • you need list methods later
  • the full collection is the real output

Laziness is helpful, but only when it matches the task.

Rules of thumb​

  • Prefer generator expressions for one-pass consumption.
  • Skip the extra list when the values are only being fed into another function.
  • Use a list comprehension when you actually need a list.
  • Favor the form that best matches how the result will be used.