diff --git a/pandas/io/common.py b/pandas/io/common.py
index d692f26ab0576..ef8b466145daa 100644
--- a/pandas/io/common.py
+++ b/pandas/io/common.py
@@ -925,7 +925,7 @@ class _BufferedWriter(BytesIO, ABC):  # type: ignore[misc]
     """
 
     @abstractmethod
-    def write_to_buffer(self):
+    def write_to_buffer(self) -> None:
         ...
 
     def close(self) -> None:
diff --git a/pandas/io/date_converters.py b/pandas/io/date_converters.py
index 077524fbee465..85e92da8c2a54 100644
--- a/pandas/io/date_converters.py
+++ b/pandas/io/date_converters.py
@@ -1,13 +1,16 @@
 """This module is designed for community supported date conversion functions"""
+from __future__ import annotations
+
 import warnings
 
 import numpy as np
 
 from pandas._libs.tslibs import parsing
+from pandas._typing import npt
 from pandas.util._exceptions import find_stack_level
 
 
-def parse_date_time(date_col, time_col):
+def parse_date_time(date_col, time_col) -> npt.NDArray[np.object_]:
     """
     Parse columns with dates and times into a single datetime column.
 
@@ -26,7 +29,7 @@ def parse_date_time(date_col, time_col):
     return parsing.try_parse_date_and_time(date_col, time_col)
 
 
-def parse_date_fields(year_col, month_col, day_col):
+def parse_date_fields(year_col, month_col, day_col) -> npt.NDArray[np.object_]:
     """
     Parse columns with years, months and days into a single date column.
 
@@ -48,7 +51,9 @@ def parse_date_fields(year_col, month_col, day_col):
     return parsing.try_parse_year_month_day(year_col, month_col, day_col)
 
 
-def parse_all_fields(year_col, month_col, day_col, hour_col, minute_col, second_col):
+def parse_all_fields(
+    year_col, month_col, day_col, hour_col, minute_col, second_col
+) -> npt.NDArray[np.object_]:
     """
     Parse columns with datetime information into a single datetime column.
 
@@ -78,7 +83,7 @@ def parse_all_fields(year_col, month_col, day_col, hour_col, minute_col, second_
     )
 
 
-def generic_parser(parse_func, *cols):
+def generic_parser(parse_func, *cols) -> np.ndarray:
     """
     Use dateparser to parse columns with data information into a single datetime column.
 
diff --git a/pandas/io/excel/_base.py b/pandas/io/excel/_base.py
index 24b881bda4805..58bb4b1a8f849 100644
--- a/pandas/io/excel/_base.py
+++ b/pandas/io/excel/_base.py
@@ -1069,7 +1069,7 @@ class ExcelWriter(metaclass=abc.ABCMeta):
     _supported_extensions: tuple[str, ...]
 
     def __new__(
-        cls,
+        cls: type[ExcelWriter],
         path: FilePath | WriteExcelBuffer | ExcelWriter,
         engine: str | None = None,
         date_format: str | None = None,
@@ -1079,7 +1079,7 @@ def __new__(
         if_sheet_exists: Literal["error", "new", "replace", "overlay"] | None = None,
         engine_kwargs: dict | None = None,
         **kwargs,
-    ):
+    ) -> ExcelWriter:
         if kwargs:
             if engine_kwargs is not None:
                 raise ValueError("Cannot use both engine_kwargs and **kwargs")
@@ -1325,7 +1325,7 @@ def cur_sheet(self):
         return self._cur_sheet
 
     @property
-    def handles(self):
+    def handles(self) -> IOHandles[bytes]:
         """
         Handles to Excel sheets.
 
@@ -1344,7 +1344,7 @@ def path(self):
         self._deprecate("path")
         return self._path
 
-    def __fspath__(self):
+    def __fspath__(self) -> str:
         return getattr(self._handles.handle, "name", "")
 
     def _get_sheet_name(self, sheet_name: str | None) -> str:
@@ -1402,10 +1402,10 @@ def check_extension(cls, ext: str) -> Literal[True]:
             return True
 
     # Allow use as a contextmanager
-    def __enter__(self):
+    def __enter__(self) -> ExcelWriter:
         return self
 
-    def __exit__(self, exc_type, exc_value, traceback):
+    def __exit__(self, exc_type, exc_value, traceback) -> None:
         self.close()
 
     def close(self) -> None:
@@ -1699,13 +1699,13 @@ def close(self) -> None:
         """close io if necessary"""
         self._reader.close()
 
-    def __enter__(self):
+    def __enter__(self) -> ExcelFile:
         return self
 
-    def __exit__(self, exc_type, exc_value, traceback):
+    def __exit__(self, exc_type, exc_value, traceback) -> None:
         self.close()
 
-    def __del__(self):
+    def __del__(self) -> None:
         # Ensure we don't leak file descriptors, but put in try/except in case
         # attributes are already deleted
         try:
diff --git a/pandas/io/excel/_openpyxl.py b/pandas/io/excel/_openpyxl.py
index 87cc07d3fd21d..c3cd3fbe9e853 100644
--- a/pandas/io/excel/_openpyxl.py
+++ b/pandas/io/excel/_openpyxl.py
@@ -33,6 +33,7 @@
 
 if TYPE_CHECKING:
     from openpyxl.descriptors.serialisable import Serialisable
+    from openpyxl.workbook import Workbook
 
 
 class OpenpyxlWriter(ExcelWriter):
@@ -79,7 +80,7 @@ def __init__(
                 self.book.remove(self.book.worksheets[0])
 
     @property
-    def book(self):
+    def book(self) -> Workbook:
         """
         Book instance of class openpyxl.workbook.Workbook.
 
diff --git a/pandas/io/feather_format.py b/pandas/io/feather_format.py
index 9813b91419060..4ecd5b7604088 100644
--- a/pandas/io/feather_format.py
+++ b/pandas/io/feather_format.py
@@ -31,7 +31,7 @@ def to_feather(
     path: FilePath | WriteBuffer[bytes],
     storage_options: StorageOptions = None,
     **kwargs,
-):
+) -> None:
     """
     Write a DataFrame to the binary Feather format.
 
diff --git a/pandas/io/parsers/base_parser.py b/pandas/io/parsers/base_parser.py
index cdd24a1194a45..531fa5400f466 100644
--- a/pandas/io/parsers/base_parser.py
+++ b/pandas/io/parsers/base_parser.py
@@ -220,7 +220,7 @@ def _validate_parse_dates_presence(self, columns: Sequence[Hashable]) -> Iterabl
             for col in cols_needed
         ]
 
-    def close(self):
+    def close(self) -> None:
         pass
 
     @final
diff --git a/pandas/io/parsers/python_parser.py b/pandas/io/parsers/python_parser.py
index ec953c9df036c..3e897f9b1334e 100644
--- a/pandas/io/parsers/python_parser.py
+++ b/pandas/io/parsers/python_parser.py
@@ -308,7 +308,11 @@ def _exclude_implicit_index(
         }, names
 
     # legacy
-    def get_chunk(self, size=None):
+    def get_chunk(
+        self, size: int | None = None
+    ) -> tuple[
+        Index | None, Sequence[Hashable] | MultiIndex, Mapping[Hashable, ArrayLike]
+    ]:
         if size is None:
             # error: "PythonParser" has no attribute "chunksize"
             size = self.chunksize  # type: ignore[attr-defined]
diff --git a/pandas/io/parsers/readers.py b/pandas/io/parsers/readers.py
index e811ace78f1f5..4b14b99f91f3f 100644
--- a/pandas/io/parsers/readers.py
+++ b/pandas/io/parsers/readers.py
@@ -493,7 +493,22 @@ class _DeprecationConfig(NamedTuple):
 }
 
 
-def validate_integer(name, val, min_val=0):
+@overload
+def validate_integer(name, val: None, min_val=...) -> None:
+    ...
+
+
+@overload
+def validate_integer(name, val: int | float, min_val=...) -> int:
+    ...
+
+
+@overload
+def validate_integer(name, val: int | None, min_val=...) -> int | None:
+    ...
+
+
+def validate_integer(name, val: int | float | None, min_val=0) -> int | None:
     """
     Checks whether the 'name' parameter for parsing is either
     an integer OR float that can SAFELY be cast to an integer
@@ -509,17 +524,18 @@ def validate_integer(name, val, min_val=0):
     min_val : int
         Minimum allowed value (val < min_val will result in a ValueError)
     """
-    msg = f"'{name:s}' must be an integer >={min_val:d}"
+    if val is None:
+        return val
 
-    if val is not None:
-        if is_float(val):
-            if int(val) != val:
-                raise ValueError(msg)
-            val = int(val)
-        elif not (is_integer(val) and val >= min_val):
+    msg = f"'{name:s}' must be an integer >={min_val:d}"
+    if is_float(val):
+        if int(val) != val:
             raise ValueError(msg)
+        val = int(val)
+    elif not (is_integer(val) and val >= min_val):
+        raise ValueError(msg)
 
-    return val
+    return int(val)
 
 
 def _validate_names(names: Sequence[Hashable] | None) -> None:
@@ -1784,7 +1800,7 @@ def __exit__(self, exc_type, exc_value, traceback) -> None:
         self.close()
 
 
-def TextParser(*args, **kwds):
+def TextParser(*args, **kwds) -> TextFileReader:
     """
     Converts lists of lists/tuples into DataFrames with proper type inference
     and optional (e.g. string to datetime) conversion. Also enables iterating
diff --git a/pandas/io/sas/sas_xport.py b/pandas/io/sas/sas_xport.py
index 500e88eb0ef76..a2e217767d1d4 100644
--- a/pandas/io/sas/sas_xport.py
+++ b/pandas/io/sas/sas_xport.py
@@ -396,7 +396,7 @@ def _read_header(self):
         dtype = np.dtype(dtypel)
         self._dtype = dtype
 
-    def __next__(self):
+    def __next__(self) -> pd.DataFrame:
         return self.read(nrows=self._chunksize or 1)
 
     def _record_count(self) -> int:
@@ -434,7 +434,7 @@ def _record_count(self) -> int:
 
         return (total_records_length - tail_pad) // self.record_length
 
-    def get_chunk(self, size=None):
+    def get_chunk(self, size=None) -> pd.DataFrame:
         """
         Reads lines from Xport file and returns as dataframe
 
diff --git a/pandas/io/sql.py b/pandas/io/sql.py
index 701642ad2cfe2..24290b8370ed2 100644
--- a/pandas/io/sql.py
+++ b/pandas/io/sql.py
@@ -14,6 +14,7 @@
 from functools import partial
 import re
 from typing import (
+    TYPE_CHECKING,
     Any,
     Iterator,
     Sequence,
@@ -50,6 +51,9 @@
 import pandas.core.common as com
 from pandas.core.tools.datetimes import to_datetime
 
+if TYPE_CHECKING:
+    from sqlalchemy import Table
+
 
 class DatabaseError(OSError):
     pass
@@ -277,7 +281,9 @@ def read_sql_table(
     if not pandas_sql.has_table(table_name):
         raise ValueError(f"Table {table_name} not found")
 
-    table = pandas_sql.read_table(
+    # error: Item "SQLiteDatabase" of "Union[SQLDatabase, SQLiteDatabase]"
+    # has no attribute "read_table"
+    table = pandas_sql.read_table(  # type: ignore[union-attr]
         table_name,
         index_col=index_col,
         coerce_float=coerce_float,
@@ -701,7 +707,7 @@ def to_sql(
     )
 
 
-def has_table(table_name: str, con, schema: str | None = None):
+def has_table(table_name: str, con, schema: str | None = None) -> bool:
     """
     Check if DataBase has named table.
 
@@ -728,7 +734,7 @@ def has_table(table_name: str, con, schema: str | None = None):
 table_exists = has_table
 
 
-def pandasSQL_builder(con, schema: str | None = None):
+def pandasSQL_builder(con, schema: str | None = None) -> SQLDatabase | SQLiteDatabase:
     """
     Convenience function to return the correct PandasSQL subclass based on the
     provided parameters.
@@ -806,7 +812,7 @@ def __init__(
     def exists(self):
         return self.pd_sql.has_table(self.name, self.schema)
 
-    def sql_schema(self):
+    def sql_schema(self) -> str:
         from sqlalchemy.schema import CreateTable
 
         return str(CreateTable(self.table).compile(self.pd_sql.connectable))
@@ -816,7 +822,7 @@ def _execute_create(self):
         self.table = self.table.to_metadata(self.pd_sql.meta)
         self.table.create(bind=self.pd_sql.connectable)
 
-    def create(self):
+    def create(self) -> None:
         if self.exists():
             if self.if_exists == "fail":
                 raise ValueError(f"Table '{self.name}' already exists.")
@@ -862,7 +868,7 @@ def _execute_insert_multi(self, conn, keys: list[str], data_iter) -> int:
         result = conn.execute(stmt)
         return result.rowcount
 
-    def insert_data(self):
+    def insert_data(self) -> tuple[list[str], list[np.ndarray]]:
         if self.index is not None:
             temp = self.frame.copy()
             temp.index.names = self.index
@@ -875,7 +881,9 @@ def insert_data(self):
 
         column_names = list(map(str, temp.columns))
         ncols = len(column_names)
-        data_list = [None] * ncols
+        # this just pre-allocates the list: None's will be replaced with ndarrays
+        # error: List item 0 has incompatible type "None"; expected "ndarray"
+        data_list: list[np.ndarray] = [None] * ncols  # type: ignore[list-item]
 
         for i, (_, ser) in enumerate(temp.items()):
             vals = ser._values
@@ -894,9 +902,7 @@ def insert_data(self):
                 mask = isna(d)
                 d[mask] = None
 
-            # error: No overload variant of "__setitem__" of "list" matches
-            # argument types "int", "ndarray"
-            data_list[i] = d  # type: ignore[call-overload]
+            data_list[i] = d
 
         return column_names, data_list
 
@@ -974,7 +980,13 @@ def _query_iterator(
 
                 yield self.frame
 
-    def read(self, coerce_float=True, parse_dates=None, columns=None, chunksize=None):
+    def read(
+        self,
+        coerce_float=True,
+        parse_dates=None,
+        columns=None,
+        chunksize=None,
+    ) -> DataFrame | Iterator[DataFrame]:
         from sqlalchemy import select
 
         if columns is not None and len(columns) > 0:
@@ -1398,7 +1410,7 @@ def read_table(
         columns=None,
         schema: str | None = None,
         chunksize: int | None = None,
-    ):
+    ) -> DataFrame | Iterator[DataFrame]:
         """
         Read SQL database table into a DataFrame.
 
@@ -1487,13 +1499,13 @@ def _query_iterator(
     def read_query(
         self,
         sql: str,
-        index_col: str | None = None,
+        index_col: str | Sequence[str] | None = None,
         coerce_float: bool = True,
         parse_dates=None,
         params=None,
         chunksize: int | None = None,
         dtype: DtypeArg | None = None,
-    ):
+    ) -> DataFrame | Iterator[DataFrame]:
         """
         Read SQL query into a DataFrame.
 
@@ -1620,7 +1632,7 @@ def check_case_sensitive(
         self,
         name,
         schema,
-    ):
+    ) -> None:
         """
         Checks table name for issues with case-sensitivity.
         Method is called after data is inserted.
@@ -1741,7 +1753,7 @@ def has_table(self, name: str, schema: str | None = None):
         insp = inspect(self.connectable)
         return insp.has_table(name, schema or self.meta.schema)
 
-    def get_table(self, table_name: str, schema: str | None = None):
+    def get_table(self, table_name: str, schema: str | None = None) -> Table:
         from sqlalchemy import (
             Numeric,
             Table,
@@ -1756,7 +1768,7 @@ def get_table(self, table_name: str, schema: str | None = None):
                 column.type.asdecimal = False
         return tbl
 
-    def drop_table(self, table_name: str, schema: str | None = None):
+    def drop_table(self, table_name: str, schema: str | None = None) -> None:
         schema = schema or self.meta.schema
         if self.has_table(table_name, schema):
             self.meta.reflect(bind=self.connectable, only=[table_name], schema=schema)
@@ -1839,7 +1851,7 @@ def __init__(self, *args, **kwargs) -> None:
         sqlite3.register_adapter(time, lambda _: _.strftime("%H:%M:%S.%f"))
         super().__init__(*args, **kwargs)
 
-    def sql_schema(self):
+    def sql_schema(self) -> str:
         return str(";\n".join(self.table))
 
     def _execute_create(self):
@@ -1847,7 +1859,7 @@ def _execute_create(self):
             for stmt in self.table:
                 conn.execute(stmt)
 
-    def insert_statement(self, *, num_rows: int):
+    def insert_statement(self, *, num_rows: int) -> str:
         names = list(map(str, self.frame.columns))
         wld = "?"  # wildcard char
         escape = _get_valid_sqlite_name
@@ -2049,7 +2061,7 @@ def read_query(
         parse_dates=None,
         chunksize: int | None = None,
         dtype: DtypeArg | None = None,
-    ):
+    ) -> DataFrame | Iterator[DataFrame]:
 
         args = _convert_params(sql, params)
         cursor = self.execute(*args)
@@ -2164,17 +2176,17 @@ def to_sql(
         table.create()
         return table.insert(chunksize, method)
 
-    def has_table(self, name: str, schema: str | None = None):
+    def has_table(self, name: str, schema: str | None = None) -> bool:
 
         wld = "?"
         query = f"SELECT name FROM sqlite_master WHERE type='table' AND name={wld};"
 
         return len(self.execute(query, [name]).fetchall()) > 0
 
-    def get_table(self, table_name: str, schema: str | None = None):
+    def get_table(self, table_name: str, schema: str | None = None) -> None:
         return None  # not supported in fallback mode
 
-    def drop_table(self, name: str, schema: str | None = None):
+    def drop_table(self, name: str, schema: str | None = None) -> None:
         drop_sql = f"DROP TABLE {_get_valid_sqlite_name(name)}"
         self.execute(drop_sql)
 
@@ -2205,7 +2217,7 @@ def get_schema(
     con=None,
     dtype: DtypeArg | None = None,
     schema: str | None = None,
-):
+) -> str:
     """
     Get the SQL db table schema for the given frame.