diff --git a/pkgs/build-support/fetchpypilegacy/default.nix b/pkgs/build-support/fetchpypilegacy/default.nix
index bcd560449916..b2715f85dd86 100644
--- a/pkgs/build-support/fetchpypilegacy/default.nix
+++ b/pkgs/build-support/fetchpypilegacy/default.nix
@@ -1,8 +1,23 @@
 # Fetch from PyPi legacy API as documented in https://warehouse.pypa.io/api-reference/legacy.html
-{ runCommand
-, lib
-, python3
+{
+  runCommand,
+  lib,
+  python3,
 }:
+let
+  inherit (lib)
+    optionalAttrs
+    fetchers
+    optional
+    inPureEvalMode
+    filter
+    head
+    concatStringsSep
+    escapeShellArg
+    ;
+
+  impureEnvVars = fetchers.proxyImpureEnvVars ++ optional inPureEvalMode "NETRC";
+in
 {
   # package name
   pname,
@@ -18,28 +33,30 @@
   name ? null,
 }:
 let
-  urls' = urls ++ lib.optional (url != null) url;
+  urls' = urls ++ optional (url != null) url;
 
-  pathParts = lib.filter ({ prefix, path }: "NETRC" == prefix) builtins.nixPath;
-  netrc_file =
-    if (pathParts != [ ])
-    then (lib.head pathParts).path
-    else "";
+  pathParts = filter ({ prefix, path }: "NETRC" == prefix) builtins.nixPath;
+  netrc_file = if (pathParts != [ ]) then (head pathParts).path else "";
 
 in
 # Assert that we have at least one URL
-assert urls' != [ ]; runCommand file
-  ({
-    nativeBuildInputs = [ python3 ];
-    impureEnvVars = lib.fetchers.proxyImpureEnvVars;
-    outputHashMode = "flat";
-    # if hash is empty select a default algo to let nix propose the actual hash.
-    outputHashAlgo = if hash == "" then "sha256" else null;
-    outputHash = hash;
-    NETRC = netrc_file;
-  }
-  // (lib.optionalAttrs (name != null) {inherit name;}))
+assert urls' != [ ];
+runCommand file
+  (
+    {
+      nativeBuildInputs = [ python3 ];
+      inherit impureEnvVars;
+      outputHashMode = "flat";
+      # if hash is empty select a default algo to let nix propose the actual hash.
+      outputHashAlgo = if hash == "" then "sha256" else null;
+      outputHash = hash;
+    }
+    // optionalAttrs (name != null) { inherit name; }
+    // optionalAttrs (!inPureEvalMode) { env.NETRC = netrc_file; }
+  )
   ''
-    python ${./fetch-legacy.py} ${lib.concatStringsSep " " (map (url: "--url ${lib.escapeShellArg url}") urls')} --pname ${pname} --filename ${file}
+    python ${./fetch-legacy.py} ${
+      concatStringsSep " " (map (url: "--url ${escapeShellArg url}") urls')
+    } --pname ${pname} --filename ${file}
     mv ${file} $out
   ''
diff --git a/pkgs/build-support/fetchpypilegacy/tests.nix b/pkgs/build-support/fetchpypilegacy/tests.nix
index b16325b96b7e..3edfd646e942 100644
--- a/pkgs/build-support/fetchpypilegacy/tests.nix
+++ b/pkgs/build-support/fetchpypilegacy/tests.nix
@@ -1,4 +1,5 @@
-{ testers, fetchPypiLegacy, ... }: {
+{ testers, fetchPypiLegacy, ... }:
+{
   # Tests that we can send custom headers with spaces in them
   fetchSimple = testers.invalidateFetcherByDrvHash fetchPypiLegacy {
     pname = "requests";