Add parser for pg_catalog.format() format strings

src/pgspot/pg_catalog/format.py

@@ -0,0 +1,69 @@
+import re
+
+# https://www.postgresql.org/docs/current/functions-string.html#FUNCTIONS-STRING-FORMAT
+#
+# Format specifiers are introduced by a % character and have the form
+#
+# %[position][flags][width]type
+# where the component fields are:
+#
+# position (optional)
+# A string of the form n$ where n is the index of the argument to print.
+# Index 1 means the first argument after formatstr. If the position is
+# omitted, the default is to use the next argument in sequence.
+#
+# flags (optional)
+# Additional options controlling how the format specifier's output is formatted.
+# Currently the only supported flag is a minus sign (-) which will cause the
+# format specifier's output to be left-justified. This has no effect unless the
+# width field is also specified.
+#
+# width (optional)
+# Specifies the minimum number of characters to use to display the format
+# specifier's output. The output is padded on the left or right (depending on
+# the - flag) with spaces as needed to fill the width. A too-small width does
+# not cause truncation of the output, but is simply ignored. The width may be
+# specified using any of the following: a positive integer; an asterisk (*) to
+# use the next function argument as the width; or a string of the form *n$ to
+# use the nth function argument as the width.
+#
+# If the width comes from a function argument, that argument is consumed before
+# the argument that is used for the format specifier's value. If the width
+# argument is negative, the result is left aligned (as if the - flag had been
+# specified) within a field of length abs(width).
+#
+# type (required)
+# The type of format conversion to use to produce the format specifier's output.
+# The following types are supported:
+# s formats the argument value as a simple string. A null value is treated as
+#   an empty string.
+# I treats the argument value as an SQL identifier, double-quoting it if
+#   necessary. It is an error for the value to be null (equivalent to quote_ident).
+# L quotes the argument value as an SQL literal. A null value is displayed as
+#   the string NULL, without quotes (equivalent to quote_nullable).
+#
+# In addition to the format specifiers described above, the special sequence %%
+# may be used to output a literal % character.
+
+
+def parse_format_string(fmt_string):
+    ret = []
+    current_index = 1
+    for i in re.findall(
+        r"(%(([1-9])$)?[-]?([1-9][0-9]*|[*]|[*][1-9]$)?([sIL]))", fmt_string
+    ):
+        # i[0] is the full match
+        # i[1] is the position specifier
+        # i[2] is the position specifier index
+        # i[3] is the width specifier
+        # i[4] is the type specifier
+        if i[2]:
+            ret.append((i[4], int(i[2])))
+        else:
+            # width may consume a function argument
+            if i[3] == "*":
+                current_index += 1
+            ret.append((i[4], current_index))
+            current_index += 1
+
+    return ret

tests/format_string_test.py

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+
+from pgspot.pg_catalog.format import parse_format_string
+
+
+def test_no_variables():
+    ret = parse_format_string("")
+    assert ret == []
+
+    ret = parse_format_string("SELECT * FROM table")
+    assert ret == []
+
+    ret = parse_format_string("%%")
+    assert ret == []
+