Warn if the requested date range isn't covered.

src/pricehist/fetch.py

@@ -1,4 +1,5 @@
 import logging
+from datetime import datetime
 
 
 def fetch(series, source, output, invert: bool, quantize: int, fmt) -> str:
@@ -8,13 +9,56 @@ def fetch(series, source, output, invert: bool, quantize: int, fmt) -> str:
             f"source start date of {source.start()}."
         )
 
-    # TODO warn if start date is today or later
-
     series = source.fetch(series)
 
+    if len(series.prices) == 0:
+        logging.warn(f"No data found for the interval [{series.start}--{series.end}].")
+    else:
+        first = series.prices[0].date
+        last = series.prices[-1].date
+        if series.start < first or series.end > last:
+            logging.warn(
+                f"Available data covers the interval [{first}--{last}], "
+                f"{_cov_description(series.start, series.end, first, last)}."
+            )
+
     if invert:
         series = series.invert()
     if quantize is not None:
         series = series.quantize(quantize)
 
     return output.format(series, source, fmt=fmt)
+
+
+def _cov_description(
+    requested_start: str, requested_end: str, actual_start: str, actual_end: str
+) -> str:
+    date_format = "%Y-%m-%d"
+    r1 = datetime.strptime(requested_start, date_format).date()
+    r2 = datetime.strptime(requested_end, date_format).date()
+    a1 = datetime.strptime(actual_start, date_format).date()
+    a2 = datetime.strptime(actual_end, date_format).date()
+    start_uncovered = (a1 - r1).days
+    end_uncovered = (r2 - a2).days
+
+    def plural(n):
+        return "" if n == 1 else "s"
+
+    if start_uncovered == 0 and end_uncovered > 0:
+        return (
+            f"starting as requested and ending {end_uncovered} "
+            f"day{plural(end_uncovered)} earlier than requested"
+        )
+    elif start_uncovered > 0 and end_uncovered == 0:
+        return (
+            f"starting {start_uncovered} day{plural(start_uncovered)} later "
+            "than requested and ending as requested"
+        )
+    elif start_uncovered > 0 and end_uncovered > 0:
+        return (
+            f"starting {start_uncovered} day{plural(start_uncovered)} later "
+            f"and ending {end_uncovered} day{plural(end_uncovered)} earlier "
+            "than requested"
+        )
+    else:
+        return "as requested"