Fix cli uncertainty package import

CHANGES

@@ -4,8 +4,10 @@ Pint Changelog
 0.25 (unreleased)
 -----------------
 
+- Fix the default behaviour for pint-convert (cli) for importing uncertainties package
 - Added mu and mc as alternatives for SI micro prefix
 - Added ℓ as alternative for liter
+- Fix the default behaviour for pint-convert (cli) for importing uncertainties package (PR #2032, Issue #2016)
 - Support permille units and `‰` symbol (PR #2033, Issue #1963)
 
 

docs/dev/pint-convert.rst

@@ -77,36 +77,39 @@ With the `uncertainties` package, the experimental uncertainty in the physical
 constants is considered, and the result is given in compact notation, with the
 uncertainty in the last figures in parentheses:
 
+The uncertainty can be enabled with `-U` (by default it is not enabled):
+
+.. code-block:: console
+
+    $ pint-convert -p 20 -U Eh eV
+    1 hartree = 27.211386245988(52) eV
+
 .. code-block:: console
 
-    $ pint-convert Eh eV
+    $ pint-convert -U Eh eV
     1 hartree = 27.21138624599(5) eV
 
 The precision is limited by both the maximum number of significant digits (`-p`)
 and the maximum number of uncertainty digits (`-u`, 2 by default)::
 
-    $ pint-convert -p 20 Eh eV
+    $ pint-convert -U -p 20 Eh eV
     1 hartree = 27.211386245988(52) eV
 
-    $ pint-convert -p 20 -u 4 Eh eV
+    $ pint-convert -U -p 20 -u 4 Eh eV
     1 hartree = 27.21138624598847(5207) eV
 
-The uncertainty can be disabled with `-U`):
-
-.. code-block:: console
-
-    $ pint-convert -p 20 -U Eh eV
-    1 hartree = 27.211386245988471444 eV
-
 Correlations between experimental constants are also known, and taken into
-account. Use `-C` to disable it:
+account if uncertainties `-U` is enabled. Use `-C` to disable it:
 
 .. code-block:: console
 
     $ pint-convert --sys atomic m_p
+    1 proton_mass = 1836.15267344 m_e
+
+    $ pint-convert -U --sys atomic m_p
     1 proton_mass = 1836.15267344(11) m_e
 
-    $ pint-convert --sys atomic -C m_p
+    $ pint-convert -U --sys atomic -C m_p
     1 proton_mass = 1836.15267344(79) m_e
 
 Again, note that results may differ slightly, usually in the last figure, from

pint/pint_convert.py

@@ -42,10 +42,10 @@
 )
 parser.add_argument(
     "-U",
-    "--no-unc",
+    "--with-unc",
     dest="unc",
-    action="store_false",
-    help="ignore uncertainties in constants",
+    action="store_true",
+    help="consider uncertainties in constants",
 )
 parser.add_argument(
     "-C",
@@ -77,7 +77,12 @@ def _set(key: str, value):
 
 
 if args.unc:
-    import uncertainties
+    try:
+        import uncertainties
+    except ImportError:
+        raise Exception(
+            "Failed to import uncertainties library!\n Please install uncertainties package"
+        )
 
     # Measured constants subject to correlation
     #  R_i: Rydberg constant
@@ -103,9 +108,14 @@ def _set(key: str, value):
             [0.00194, 0.97560, 0.98516, 0.98058, 1.0, 0.51521],  # m_p
             [0.00233, 0.52445, 0.52959, 0.52714, 0.51521, 1.0],
         ]  # m_n
-        (R_i, g_e, m_u, m_e, m_p, m_n) = uncertainties.correlated_values_norm(
-            [R_i, g_e, m_u, m_e, m_p, m_n], corr
-        )
+        try:
+            (R_i, g_e, m_u, m_e, m_p, m_n) = uncertainties.correlated_values_norm(
+                [R_i, g_e, m_u, m_e, m_p, m_n], corr
+            )
+        except AttributeError:
+            raise Exception(
+                "Correlation cannot be calculated!\n  Please install numpy package"
+            )
     else:
         R_i = uncertainties.ufloat(*R_i)
         g_e = uncertainties.ufloat(*g_e)
@@ -160,6 +170,7 @@ def _set(key: str, value):
 
 
 def convert(u_from, u_to=None, unc=None, factor=None):
+    prec_unc = 0
     q = ureg.Quantity(u_from)
     fmt = f".{args.prec}g"
     if unc:
@@ -171,7 +182,8 @@ def convert(u_from, u_to=None, unc=None, factor=None):
     if factor:
         q *= ureg.Quantity(factor)
         nq *= ureg.Quantity(factor).to_base_units()
-    prec_unc = use_unc(nq.magnitude, fmt, args.prec_unc)
+    if args.unc:
+        prec_unc = use_unc(nq.magnitude, fmt, args.prec_unc)
     if prec_unc > 0:
         fmt = f".{prec_unc}uS"
     else: