Make pygments dependency optional

Closes #5555

8 files changed, 55 insertions, 27 deletions

diff --git a/doc/changelog.asciidoc b/doc/changelog.asciidoc
index face04c4d..c338d1545 100644
--- a/doc/changelog.asciidoc
+++ b/doc/changelog.asciidoc
@@ -44,6 +44,10 @@ Major changes
   still relying on it. The `cssutils` project is also dead upstream, with its
   repository being gone after Bitbucket
   https://bitbucket.org/blog/sunsetting-mercurial-support-in-bitbucket[removed Mercurial support].
+- The (formerly required) `pygments` dependency is now optional. It is only
+  used when using `:view-source` with QtWebKit, or when forcing it via
+  `:view-source --pygments` on QtWebEngine. If it is unavailable, an
+  unhighlighted fallback version of the page's source is shown.
 - TODO: The former dependency on the `pkg_resources` module (part of the
   `setuptools` project) got dropped.
 - A new dependency on the `importlib_resources` module got introduced for
diff --git a/doc/help/commands.asciidoc b/doc/help/commands.asciidoc
index eb8e4925d..3564c9766 100644
--- a/doc/help/commands.asciidoc
+++ b/doc/help/commands.asciidoc
@@ -1515,6 +1515,7 @@ Show the source of the current page in a new tab.
 * +*-p*+, +*--pygments*+: Use pygments to generate the view. This is always the case for QtWebKit. For QtWebEngine it may display
  slightly different source.
  Some JavaScript processing may be applied.
+ Needs the optional Pygments dependency for highlighting.
 
 
 [[window-only]]
diff --git a/misc/requirements/requirements-qutebrowser.txt-raw b/misc/requirements/requirements-qutebrowser.txt-raw
index 2d527aeef..03e6324f6 100644
--- a/misc/requirements/requirements-qutebrowser.txt-raw
+++ b/misc/requirements/requirements-qutebrowser.txt-raw
@@ -1,10 +1,12 @@
 Jinja2
-Pygments
 pyPEG2
 PyYAML
-colorama
 attrs
-adblock  # Optional, for improved adblocking
 importlib-resources
 
+## Optional dependencies
+Pygments  # For :view-source --pygments or on QtWebKit
+colorama  # Colored log output on Windows
+adblock  # Improved adblocking
+
 #@ markers: importlib-resources python_version<"3.9"
diff --git a/qutebrowser/browser/browsertab.py b/qutebrowser/browser/browsertab.py
index dda7c7720..c62c75d48 100644
--- a/qutebrowser/browser/browsertab.py
+++ b/qutebrowser/browser/browsertab.py
@@ -38,14 +38,10 @@ if TYPE_CHECKING:
     from PyQt5.QtWebKitWidgets import QWebPage
     from PyQt5.QtWebEngineWidgets import QWebEngineHistory, QWebEnginePage
 
-import pygments
-import pygments.lexers
-import pygments.formatters
-
 from qutebrowser.keyinput import modeman
 from qutebrowser.config import config
 from qutebrowser.utils import (utils, objreg, usertypes, log, qtutils,
-                               urlutils, message)
+                               urlutils, message, jinja)
 from qutebrowser.misc import miscwidgets, objects, sessions
 from qutebrowser.browser import eventfilter, inspector
 from qutebrowser.qt import sip
@@ -177,30 +173,52 @@ class AbstractAction:
             raise WebTabError("{} is not a valid web action!".format(name))
         self._widget.triggerPageAction(member)
 
-    def show_source(
-            self,
-            pygments: bool = False  # pylint: disable=redefined-outer-name
-    ) -> None:
+    def show_source(self, pygments: bool = False) -> None:
         """Show the source of the current page in a new tab."""
         raise NotImplementedError
 
+    def _show_html_source(self, html: str) -> None:
+        """Show the given HTML as source page."""
+        tb = objreg.get('tabbed-browser', scope='window', window=self._tab.win_id)
+        new_tab = tb.tabopen(background=False, related=True)
+        new_tab.set_html(html, self._tab.url())
+        new_tab.data.viewing_source = True
+
+    def _show_source_fallback(self, source: str) -> None:
+        """Show source with pygments unavailable."""
+        html = jinja.render(
+            'pre.html',
+            title='Source',
+            content=source,
+            preamble="Note: The optional Pygments dependency wasn't found - "
+            "showing unhighlighted source.",
+        )
+        self._show_html_source(html)
+
     def _show_source_pygments(self) -> None:
 
         def show_source_cb(source: str) -> None:
             """Show source as soon as it's ready."""
-            # WORKAROUND for https://github.com/PyCQA/pylint/issues/491
-            # pylint: disable=no-member
-            lexer = pygments.lexers.HtmlLexer()
-            formatter = pygments.formatters.HtmlFormatter(
-                full=True, linenos='table')
-            # pylint: enable=no-member
-            highlighted = pygments.highlight(source, lexer, formatter)
-
-            tb = objreg.get('tabbed-browser', scope='window',
-                            window=self._tab.win_id)
-            new_tab = tb.tabopen(background=False, related=True)
-            new_tab.set_html(highlighted, self._tab.url())
-            new_tab.data.viewing_source = True
+            try:
+                import pygments
+                import pygments.lexers
+                import pygments.formatters
+            except ImportError:
+                # Pygments is an optional dependency
+                self._show_source_fallback(source)
+                return
+
+            try:
+                lexer = pygments.lexers.HtmlLexer()
+                formatter = pygments.formatters.HtmlFormatter(
+                    full=True, linenos='table')
+            except AttributeError:
+                # Remaining namespace package from Pygments
+                self._show_source_fallback(source)
+                return
+
+            html = pygments.highlight(source, lexer, formatter)
+            self._show_html_source(html)
 
         self._tab.dump_async(show_source_cb)
 
diff --git a/qutebrowser/browser/commands.py b/qutebrowser/browser/commands.py
index 18777e250..fda645416 100644
--- a/qutebrowser/browser/commands.py
+++ b/qutebrowser/browser/commands.py
@@ -1343,6 +1343,7 @@ class CommandDispatcher:
                       the case for QtWebKit. For QtWebEngine it may display
                       slightly different source.
                       Some JavaScript processing may be applied.
+                      Needs the optional Pygments dependency for highlighting.
         """
         tab = self._current_widget()
         try:
diff --git a/qutebrowser/html/pre.html b/qutebrowser/html/pre.html
index cfcfad359..5fb9c7f40 100644
--- a/qutebrowser/html/pre.html
+++ b/qutebrowser/html/pre.html
@@ -1,6 +1,9 @@
 {% extends "base.html" %}
 {% block content %}
 {{ super() }}
+{% if preamble is defined %}
+    <p>{{ preamble }}</p>
+{% endif %}
 <pre>
 {{ content }}
 </pre>
diff --git a/qutebrowser/misc/earlyinit.py b/qutebrowser/misc/earlyinit.py
index 413e9d111..936716078 100644
--- a/qutebrowser/misc/earlyinit.py
+++ b/qutebrowser/misc/earlyinit.py
@@ -228,7 +228,6 @@ def check_libraries():
         'pkg_resources': _missing_str("pkg_resources/setuptools"),
         'pypeg2': _missing_str("pypeg2"),
         'jinja2': _missing_str("jinja2"),
-        'pygments': _missing_str("pygments"),
         'yaml': _missing_str("PyYAML"),
         'attr': _missing_str("attrs"),
         'PyQt5.QtQml': _missing_str("PyQt5.QtQml"),
diff --git a/setup.py b/setup.py
index 137b514ca..fbbc37153 100755
--- a/setup.py
+++ b/setup.py
@@ -71,7 +71,7 @@ try:
         entry_points={'gui_scripts':
                       ['qutebrowser = qutebrowser.qutebrowser:main']},
         zip_safe=True,
-        install_requires=['pypeg2', 'jinja2', 'pygments', 'PyYAML', 'attrs',
+        install_requires=['pypeg2', 'jinja2', 'PyYAML', 'attrs',
                           'importlib_resources>=1.1.0; python_version < "3.9"'],
         python_requires='>=3.6',
         name='qutebrowser',