Contents

• plxpython3: the Python dialect
  • Setup
  • Function basics
    • Local variables and types
  • Control flow
    • if / elif / else
    • Loops
    • Loop labels
  • Working with data
    • Iterating a query
    • Iterating an array
    • Fetching one row
    • Running SQL
    • Cursors
    • Diagnostics
    • Set-returning functions
  • Errors
    • Raising
    • Handling
    • Assertions
  • Expressions
  • Building strings in a loop
  • Trigger functions
  • Semantic differences
  • Not supported

plxpython3: the Python dialect

plxpython3 lets you write PostgreSQL functions with Python syntax. At CREATE FUNCTION time plx transpiles the body to plpgsql and stores the plpgsql in pg_proc.prosrc. The function runs on the standard plpgsql interpreter.

The language name is plxpython3, so it does not collide with the native PL/Python (plpython3u). plxpython3 is a trusted language, unlike plpython3u.

Setup

CREATE EXTENSION plx;

Function basics

Blocks are defined by indentation, as in Python. A compound-statement header ends with : and its body is indented. A scalar function ends with an explicit return.

CREATE FUNCTION add(a int, b int) RETURNS int LANGUAGE plxpython3 AS $$
return a + b
$$;

Function arguments are referenced by name. Any SQL expression is valid, because expressions are passed through to plpgsql and SQL unchanged.

Local variables and types

Assignment creates a local. plx infers the type from the first value when it is an integer, numeric, text, or boolean literal. Otherwise annotate the variable with #:: type.

n = 42            # inferred integer
label = "count"   # inferred text
amount #:: numeric
amount = principal * rate

The annotation text is emitted verbatim, so %TYPE and %ROWTYPE work:

e #:: emp%ROWTYPE
s #:: emp.sal%TYPE

A constant uses the const suffix on the annotation:

pi = 3.14159 #:: numeric const

Control flow

if / elif / else

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "F"

Loops

for ... in range(...) is an integer loop. range(n) is 0 to n-1, range(a, b) is a to b-1, range(a, b, step) adds a step.

for i in range(1, n + 1):
    total = total + i

while n > 0:
    n = n - 1

continue and break map to CONTINUE and EXIT. pass is a no-op.

Loop labels

outer: for i in range(n):
    for j in range(n):
        if i + j >= limit:
            break outer

Working with data

Iterating a query

for row in query(f"SELECT id, amount FROM orders WHERE grp = {g}"):
    total = total + row.amount

The row variable is a record. Field access is row.col. For bind parameters or non-literal SQL, pass extra arguments: query(sql, a).

Iterating an array

v #:: int
for v in values:
    total = total + v

The loop variable must be annotated with its element type.

Fetching one row

u = fetch_one(f"SELECT id, name FROM users WHERE id = {uid}")
return u.name

fetch_one returns all-NULL on no row. fetch_one! raises on zero or more than one row.

Running SQL

perform(f"UPDATE counters SET n = n + 1 WHERE id = {cid}")
execute("INSERT INTO t(msg) VALUES ($1)", note)
return_query("SELECT id FROM vip ORDER BY id")   # set-returning function

Use bind arguments in execute for untrusted input.

Cursors

c = open_cursor("SELECT v FROM t ORDER BY v")
row = fetch_from(c)
while found():
    total = total + row.v
    row = fetch_from(c)
close_cursor(c)

move_cursor(c) and move_cursor(c, n) map to MOVE.

Diagnostics

rc = row_count()   # GET DIAGNOSTICS rc = ROW_COUNT
found()            # the FOUND variable

Set-returning functions

CREATE FUNCTION squares(n int) RETURNS SETOF int LANGUAGE plxpython3 AS $$
for i in range(1, n + 1):
    return_next(i * i)
return
$$;

Errors

Raising

raise ValueError(f"negative: {v}")

The call form raise("notice", "message") emits a RAISE at the named level (notice, warning, info, log, debug, exception). A bare raise inside an except handler re-raises.

Handling

try:
    execute("INSERT INTO uniq(id) VALUES ($1)", k)
except PG::UniqueViolation as e:
    raise("notice", f"dup on {e.constraint}")
finally:
    perform("INSERT INTO log(msg) VALUES ('done')")

except Exception as e maps to WHEN OTHERS. To catch a specific condition, name it with the PG:: spelling (except PG::UniqueViolation as e), which maps to unique_violation. Accessors: e.message to SQLERRM, e.sqlstate to SQLSTATE, and e.detail, e.hint, e.constraint, e.column, e.table, e.schema, e.datatype to the matching GET STACKED DIAGNOSTICS fields.

Assertions

assert n > 0, "must be positive"

Expressions

• Interpolation: f-strings, f"total is {amount}". Plain strings do not interpolate. {{ and }} are literal braces.
• Comparison: == and != map to = and <>. x is None becomes x IS NULL and x is not None becomes x IS NOT NULL.
• Boolean: and, or, not map to AND, OR, NOT.
• Ternary: Python’s a if c else b is not supported; use an explicit if.
• None becomes NULL.

Building strings in a loop

Concatenating onto a string in a loop is slow in plpgsql: s := s || 'x' is O(n²) because text is immutable and each step copies the whole string. Use += on a string variable, which plx lowers to its string builder (plx_strbuild):

s = "" #:: text
for row in query("SELECT name FROM t ORDER BY id"):
    s += row.name
    s += ","
return s

On PostgreSQL 18 this is amortized O(1) per append. On PostgreSQL 13 to 17 it is correct but not accelerated (the in-place optimization needs a PostgreSQL 18 feature). Note that += is treated as string append only when the variable is a string; on a numeric variable it is numeric addition.

Trigger functions

A function returning trigger can be used as a trigger. Assign to NEW fields and return NEW:

CREATE FUNCTION stamp() RETURNS trigger LANGUAGE plxpython3 AS $$
NEW.tag = f"row {NEW.id}"
return NEW
$$;

NEW, OLD, and the TG_ variables are available. Assigning to a record field (NEW.col = e) or an array element is supported.

Semantic differences

These are intentional. plx pins semantics to SQL and plpgsql.

• Decimal literals infer numeric, not a floating-point type.
• Comparisons use SQL three-valued logic: == and != map to = and <>, so a comparison involving NULL is unknown. Use is None / is not None (which become IS NULL / IS NOT NULL) to test for NULL. A positive if/while condition treats NULL as false.
• Integer division follows SQL rules, not Python’s / (float) and // (floor). Division and modulo truncate toward zero: -7 // 2 is -4 in Python but the SQL result here is -3, and -7 % 2 is 1 in Python but -1 here.
• Interpolating a NULL in an f-string propagates it, so the whole string is NULL. It is neither the Python string "None" nor an empty string. In a message being built for raise the value falls back to an empty string instead, so one NULL cannot swallow the message.
• Comparisons use SQL type resolution. 1 == "1" compares an integer to a string literal, which SQL coerces and treats as equal; in Python it is false.
• Python truthiness is not emulated. A condition must be a boolean expression. A non-boolean condition is an error reported by plpgsql when the function runs.
• Locals are function-scoped, matching Python function scope.

Not supported

Rejected at CREATE FUNCTION time with a line number:

• def, class, lambda, decorators, comprehensions, generators.
• import, modules, and the Python standard library.
• Tuples, lists, dicts, and sets as general values.
• match/case.
• The conditional expression a if c else b.
• Per-block local scope (locals are function-scoped by design).

See PARITY.md for the full plpgsql construct matrix and ARCHITECTURE.md for how plx maps to the plpgsql engine.