Fix the case where there is no swap

In cases where there is no swap we don't display a percentage.
The case with zero memory is also covered event if it's not realistic it
doesn't cost much to do it.

Issue #318

pgactivity/types.py

@@ -696,11 +696,17 @@ def default(cls) -> "SwapInfo":
         return cls(0, 0, 0)
 
     @property
-    def pct_used(self) -> Pct:
+    def pct_used(self) -> Optional[Pct]:
+        if self.total == 0:
+            # account for the zero swap case (#318)
+            return None
         return Pct(self.used * 100 / self.total)
 
     @property
-    def pct_free(self) -> Pct:
+    def pct_free(self) -> Optional[Pct]:
+        if self.total == 0:
+            # account for the zero swap case (#318)
+            return None
         return Pct(self.free * 100 / self.total)
 
 
@@ -716,15 +722,21 @@ def default(cls) -> "MemoryInfo":
         return cls(0, 0, 0, 0)
 
     @property
-    def pct_used(self) -> Pct:
+    def pct_used(self) -> Optional[Pct]:
+        if self.total == 0:
+            return None
         return Pct(self.used * 100 / self.total)
 
     @property
-    def pct_free(self) -> Pct:
+    def pct_free(self) -> Optional[Pct]:
+        if self.total == 0:
+            return None
         return Pct(self.free * 100 / self.total)
 
     @property
-    def pct_bc(self) -> Pct:
+    def pct_bc(self) -> Optional[Pct]:
+        if self.total == 0:
+            return None
         return Pct(self.buff_cached * 100 / self.total)
 
 

tests/test_views.txt

@@ -184,6 +184,23 @@ PostgreSQL 9.6 - localhost - tester@host:5432/postgres - Ref.: 10s - Duration mo
 ...        width=200)
 PostgreSQL 9.6 - localhost - tester@host:5432/postgres - Ref.: 2s - Duration mode: transaction - Min. duration: 1.2s
 
+>>> sysinfo = SystemInfo(
+...     MemoryInfo(used=0, buff_cached=0, free=0, total=0),
+...     SwapInfo(used=0, free=0, total=0),
+...     LoadAverage(avg1=0, avg5=0, avg15=0),
+...     io_read=IOCounter(0,0),
+...     io_write=IOCounter(0,0),
+...     max_iops=0)
+>>> ui.toggle_system_info_in_header()
+>>> header(term, ui, host=host, server_information=serverinfo,
+...        system_info=sysinfo, pg_version="PostgreSQL 9.6",
+...        width=200)
+PostgreSQL 9.6 - localhost - tester@host:5432/postgres - Ref.: 2s - Duration mode: transaction - Min. duration: 1.2s
+ * Mem.: 0B total ⋅ 0B (-) free ⋅ 0B (-) used ⋅ 0B (-) buff+cached
+   Swap: 0B total ⋅ 0B (-) free ⋅ 0B (-) used
+   IO: 0/s max iops ⋅ 0B/s - 0/s read ⋅ 0B/s - 0/s write
+   Load average: 0 0 0
+
 Tests for processes_rows()
 --------------------------